This sent me down a rabbit hole; interesting stuff.
The way I'm understanding this: the way an image is rendered depends on lots of different factors like browser, OS, graphics card etc, plus other bits like fonts and anti-aliasing settings. Each persons set up is unique so an image rendering on that browser on that device will be unique. So to fingerprint and track someone, they get your browser to generate a reference image then extract the details of the created pixels from the memory and generate an MD5 hash which is then unique to your browser. That's your fingerprint and every time a site generates the reference image it produces the same MD5 hash. That is then used to track you.
So an anti-fingerprinting technique is to throw in a very subtle randomness to the colours generated in the image, which results in a unique MD5 each time the test is run making it useless for people tracking you (you are essentially a "new" browser every time a site tries to fingerprint you). So if you have #000000 for black, instead it may randomise to #000003 one time, and #002000 next. It's a very subtle variance on the colours so won't be readily perceptible but on images rendered and shown to you this would create very subtle noise. Hence the warning for a graphics tool; the makers are aware this effects how their tool works and are warning you incase you notice the results.