Silver Strength!TwiDasH7n2 No.6164
File: 1353684094968.png (621.05 KB, 1280x720, Rarity face.png)
PNGs are lossless. If you copy a picture into MS Paint and save it as a png, the new picture will be the exactly the same. However, the file for the picture may be different, because there are multiple ways to encode the same image as a png. GIMP offers ten different levels of compression, depending on whether you want a smaller filer size, or you want the image to take less processing power to open.
According to Wikipedia, MS Paint's is kinda crappy, and results in files that take up a needlessly large amount of space.
…anyway, I presume the code compares the file that encodes the picture rather than opening the file and comparing the resulting picture. So a different encoding should count as a different picture.