I am currently testing a couple of desktop applications and server-side scripts for image size reduction and compression. I have always heard that the optimal comprise for size and quality is at about 70% compression. I ran the exact same picture through a program and could see no apparent difference for a web image between the 50-90% ranges. This 3.1 mega pixel picture, original image, in JPG format had a lot of similar colors. In each case I reduced it and compressed it at the same time from 2160 pixels wide to 600 pixels wide keeping the aspect ratio the same. In each case I started from the original image to avoid the double compression factor. My question is what type of picture would test the theory the best? More colors? Sharp lines? Close up? Colbyt
Im not exactly sure what your going for, but I did read this So if I read it right, you might want to try a grayscale image with this theory. Here is the page that the quote was off of