I ran an image I grabbed off Google through TinyPng's website. The image was compressed, something like 21%. Out of curiosity I put it in again and it was compressed again, by a much lesser value. I kept going, in total 19 times, before the files was no longer getting smaller. Leaving this broken looking version of a cat.
I then grabbed another picture of a cat off Google and ran it through TinyPng, the very first time showed there was 0% compression. Yet there was no artifacts or discolorations as shown in the first picture.
Why was the second image not able to be compressed? Why was the first image stopped from compressing more to the point of being a bunch of garbled pixels?