mjhjmtu

This is my code:

defineexpandable[1]background
 directlua
 local cases = 
 "noise-blue.png",
 "noise-yellow.png",
 "noise-red.png",
 "noise-green.png"
 

 tex.sprint(cases[#1])
 


starttext
 background1 % (1)
 externalfigure[noise-blue.png][width=4cm] % (2)
 externalfigure[background1][width=4cm] % (3)
stoptext

(1) gives me noise-blue.png in the output.
(2) correctly renders the image noise-blue.png.
(3) gives a gray box with

name: noise-blue.png
file: noise-blue.png
state: unknown

Why doesn't it render the image?

You add spaces in the output of your command at the begin and end of the definition.

defineexpandable[1]Background
 directlua
 local cases = 
 "noise-blue.png",
 "noise-yellow.png",
 "noise-red.png",
 "noise-green.png"
 
 tex.sprint(cases[#1])
 


starttext
“Background1”
stoptext

To get rid of the spaces you have to add a comment sign after the braces.

defineexpandable[1]Background%
 directlua
 local cases = 
 "noise-blue.png",
 "noise-yellow.png",
 "noise-red.png",
 "noise-green.png"
 
 tex.sprint(cases[#1])
 %


starttext
“Background1”
stoptext

You can also use the texdefinition environment to create the command which doesn’t convert the end of a line into a space.

starttexdefinition Background #1
 startlua
 local cases = 
 "noise-blue.png",
 "noise-yellow.png",
 "noise-red.png",
 "noise-green.png"
 
 context(cases[#1])
 stoplua
stoptexdefinition

Alternative solution

A different way to create a index for your images is the setvariables which you can use to assign a key to each image. The graphics can than be accessed with the getvariable command.

setvariables
 [background]
 [1=noise-blue.png,
 2=noise-yellow.png,
 3=noise-red.png,
 4=noise-green.png]

starttext
“getvariablebackground1”
stoptext

As an alternative to Wolfgang's answer and as alternative to your Lua solution, you might want to consider using useexternalfigure which allows you to name figures.

useexternalfigure[background:1][noise-blue.png]
useexternalfigure[background:2][noise-yellow.png]
useexternalfigure[background:3][noise-red.png]
useexternalfigure[background:4][noise-green.png]

starttext
 externalfigure[background:1][width=4cm]
stoptext