For a crispness test, you could try a test image like this
http://www.musigma.nl/download/testp...n_768x1024.gif ( originally posted here
https://www.mobileread.com/forums/sho...6&postcount=23 )
You should be able to make out the individual vertical/horizontal lines, the circles, and the grid around the circles, all 1 pixel wide on a 768x1024 display.
That is, unless the Kindle doesn't display images full screen. If there's a status bar or something you'll have to crop the image by whatever many pixels the Kindle wastes on that.