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The way to "double-click" on a file from the command line is xdg-open.

If you're on Gnome (probably, if you're using Nautilus), you can use eog directly, or any other image program (feh is quite good).

feh <image-name> 

If you want to consult image-name file easilly.

If you happen to have installed imagemagick, you can use its very handy display command-line tool.

display path/to/picture.png

I usually run python -m SimpleHTTPServer 8080 from a folder containing the image and view it from the browser at localhost:8080/path/to/image.jpg. Works great to browse and look at different image and text files in that same directory.

Edit: This method also works for remote boxes. Just run the python server as above, and if the ports are open you can connect to myserver.com:8080/path/to/image.jpg. If the server does not expose ports, you can set up an ssh tunnel with ssh me@myserver.com -N -L localhost:8080:localhost:8080 and then view the images from your local browser at localhost:8080/path/to/image.jpg. Note this http interface is much faster than ssh -X.

I have created a tool that uses various unicode block graphics characters to improve the resolution (compared to the canonical 2 pixel per characterapproach taken by other solutions posted here). It also automatically displays images in a thumbnail mode if multiple images are provided as input.

Installation:

git clone https://github.com/stefanhaustein/TerminalImageViewer.git
cd TerminalImageViewer/src/main/cpp
make
sudo make install

Usage:

tiv <image(s)>

More details at the github project: https://github.com/stefanhaustein/TerminalImageViewer

Example screenshot:

I guess feh would be the one you are looking for.

Although popping out a terminal-like window for picture display as mplayer does for video playing, Feh is still considered as a terminal-based picture viewer due to its command-intensive control style.

If you're on OS X, iTerm 2 supports displaying inline images:

As you can imagine, this can be highly useful when you just want to display an image without going in and opening up Preview.app or having to open the Finder to the image and hitting spacebar on the image.

The feature is documented at iTerm2's feature page

If you're in pure terminal with framebuffer activated, I'd recommend fbi. It's always in my after-install to do list.

Otherwise, eog is the default picture opener in Gnome, as Kevin mentioned. Also, Shotwell seems to open pretty fast for me... I'm pretty sure there are hundreds. :-)

Surprised that the Terminology terminal emulator is not among the answers. As easy as tyls and then clicking on an image:

or someting like tycat image.png, for example:

Plus a lot more. An old video, almost 3 years ago, demonstrating Terminology 0.3: https://www.youtube.com/watch?v=ibPziLRGvkg#action=share.

w3m's imgdisplay library is able to display images directly. It only works on some terminal emulators such as Xterm and URxvt.

ranger is a file browser that makes use of the library to display previews. Add this to ranger's configuration file to enable the feature:

set preview_images true

Actually, there is a way to show a PNG file in a terminal in text-mode, using picture-tube:

You can look at the project on GitHub.

The image is of course an approximation of the actual image, given the fact that the smallest pixel is a character - try to have a look at the result in a text editor, you will see a huge number of color codes.

You can install the tool like this (assuming you have already installed node/npm):
npm install -g picture-tube

Another great terminal-based picture viewer is fim, which is a improved fbi with a vim-like control system.

This is a 4 year-old-question but I found that people still look at it. So, today I would like to answer my own question specifically only for iTerm2.

At first time I asked this question. The term of "fastest" I was thinking of viewing image as fast as cat command displaying text. So, today I found the answer on iTerm2, which are the two handy commands name imgcat and imgls.

To use them, the first let's install iterm2-nigthly:

$ brew cask install iterm2-nightly

then visit the website:

https://github.com/olivere/iterm2-imagetools

and install imgcat and imgls commands:

$ go install github.com/olivere/iterm2-imagetools/cmd/imgcat
$ go install github.com/olivere/iterm2-imagetools/cmd/imgls

So, I really love them and also hope this will be useful for you all!

There is a tool here called catimg that works really well:

https://github.com/posva/catimg

Also has deb packages on https://launchpad.net/ubuntu/+source/catimg

pxl is a pretty cool option which hasn't been mentioned yet. Similar to picture-tube which was previously posted in that it shows the image directly in the terminal although it supports more image formats.

I just use the Chrome webrowser, which supports jpg, svg, png, ico, animated gif... basically everything I'm likely to need:

google-chrome ./myphoto.jpg

If you prefer Firefox:

firefox ./myphoto.jpg

In linux you can install gnome-open.

# In Debian flavors

sudo apt-get install gnome-open

Then create an alias:

alias open='gnome-open'

Then you can:

open file.pdf
open file.jpg
etc

From the terminal? Why not in the terminal: cacaview might be the fastest. By default it's fastest if the terminal has no graphics.

In Debian: sudo apt-get install caca-utils

No dependencies, one C file, one header file: imcat

Works on linux, macos, windows.
Automatically scales to terminal width, with quality down-sampling.

Remark: seriously? Down-voted? Next time, maybe explain in comment why.

I just found this page, and it is still very relevant today. OP didn't really specify if they want X or fb viewer.

I use a small utility called pho: http://www.shallowsky.com/software/pho/

Needs X, so call from command line in a terminal window. Views many picture formats, gif and jpg for sure... some others as well.

Can do wildcard globbing from command line: eg. "$ pho img00??.jpg" will view all files that match in order. Space goes forward through list and backspace goes back. Its quick loading individual images -- hold down space and you fly through them.

Very fast if you want to look through a bunch of pictures, flag ones of interest, rotate, delete unwanted... I keep coming back to it.

From the web page above:

pho (pronounced like the first syllable in "photo") is a lightweight program for viewing large numbers of images quickly, rotating or deleting some, and making notes about what to do with each image -- for instance, for going through hundreds of images after uploading them from a digital camera.

Even tough there are great answers given, my issue was to view images in terminal itself, without openning any graphical application, so i wrote my own little script. I only needed to identify the image and did not need al the details of it and did not want to download any relatively big packet, if anyone needs they can also use it:
https://github.com/Deniz97/terminal-image-viewer

I have a shell alias for macOS called preview that can (1) display an image inline (-i or --inline option), either from stdin or specified filepath(s), and (2) just open an image in Apple's Preview app (without any option), the latter also either from stdin or filepath(s), e.g.:

cat image.jpg | preview

preview -i image.png

Inline display only works with the iTerm imgcat alias, so you need iTerm installed, running, and frontmost. But preview checks if all that is true.

Online here: https://github.com/JayBrown/Tools/blob/master/Aliases/preview

The kitty terminal can display inside the terminal with the icat "kitten"

E.g.

kitty +kitten icat /path/to/image

Example:

The kitty documentation suggests creating an alias, e.g.

alias icat="kitty +kitten icat"

So that then you can simply do:

icat /path/to/image

If using gnome it is as simple as typing

$ gnome-open /path/to/photo

and it opens the image using the default image viewer that you have in your machine.