MArcomage

This is probably the most trivial issue ever reported, but... Some of the card pictures you're using have invalid ICC profiles. According to libpng: "iCCP: profile 'ICC profile': 1C5Dh: invalid length". I know for sure that the files card_435.png and card_261.png are affected by this, but I have tested only a few so far.

To auto-fix this, you can use a program that outputs correct data, like ImageMagick or such:
convert card_435.png card_435f.png
mv card_435f.png card_435.png
You could do that on the entire image database, since PNGs are lossless and thus those that are already fine won't be affected.

Thanks for reporting, I'll look into it. The images were originally in JPG format, but we converted them with Gimp automated script to PNG. Maybe there was an issue with script configuration.

Some of the other files with invalid profiles: card_390, card_146, card_107. And I just checked, fixing them actually results in a pretty hefty size reduction, up to 20%, so it's actually worthwhile to fix it.