What can *.itc files contain when not PNG or JPG?
Well thanks @beroe for the link. 

After some digging I wrote a little documentation of .itc files. 

.itc files are actually wrapper of images that are either PNG, JPG or ARGB raw files. .itc files can hold multiple images with different size. Most of the time there are 3 images of (128*128, 256*256 &amp; 400*400).  

I wrote a little bash script that convert all .itc files into jpg or png and puts them in ~/Music/iTunes/Album Artwork/Images.