This one is all made in Java, no extra dependencies. Just open it to run.
Get it now!
It will ask you for the source image, destination file, the ComputerCraft image size (a full 8x6 monitor is 82x40) and whether to stretch or fit. Use pastebin to get the destination file in your computers and paintutils to draw it:
image = paintutils.loadImage("imageFileNameHere") paintutils.drawImage(image, 1, 1)
Simple as that.
Disclaimer: Uses color approximation algorithms stolen from WorldEdit, in turn stolen from a StackOverflow question.