Sorry to revive this thread but i saw it from a signature someplace, got interested and wanted to tell you how to show differences.
Here's a DIFF of the last two images posted.
Basically, any layer capable image editor will let you overlay the original image with the compressed one. Then you set layer blending to "difference" and you get to see just how much difference there is between the two.
The more saturated the differences are, the more of a color difference there is. The brighter the differences are, the more brightness difference there is.
Different colors doesn't really matter that much. But the more colors tend towards red, the bigger the HUE difference is. As in, when there's red pixels, the original color might have been green but are now red or blue in the compressed image. Changes like that can really stand out.
And a comparison with transparency neutralized (put black layers behind each variant, merged variants with black and then difference) shows that practically ALL the difference is in the transparency.
Finally, if you really want to show even the minutest of differences you can take the above image and increase saturation to the max on it and add some brightness to it.
Again, the source data is just the difference per pixel between the original and modified data.
But this highlights any differences, even the ones one can't see even if they had eagle eyes.
... Just wanted to show that.