After installing 7zip you should be able to right click on the file and select to extract it. By adding it to an archive it compresses away all the empty data. An ISO image is always the size of the CD/DVD even if you only have 1MB of data on it. Save it as a text file as something like “zip to 7z.bat”, put it in your ROM folder and run it. If you need to open one on a mobile device, free apps like iZip (iOS) and 7Zipper (Android) should work.
The home site for development versions (i.e., may be buggy or subject to change or include experimental features) is, and the place to go for questions about the library is the png-mng-implement mailing list. It supports almost all PNG features, is extensible, and has been extensively tested for over 23 years. libpng is the official PNG reference library.
