Well, I have unpacked the files. A few of them can be opened with an image viewer, and I can usually see the image file as a preview image in the file browser. If I want to open the broken files, nothing is displayed, whether I open it with an image viewer, browser or GIMP. The last also indicates that it is not a PNG format. 

EDIT: Under Windows 10 its working. Btw nice Set! <3 Before I tried this in Linux Mint.