The 2GB (full-res) version offers higher image quality but contains the same content as the 1GB version. The full-res version has a more elaborate installation procedure, it comes with a txt file that explains what you need to do.
A zip file is essentially a folder that contains multiple files, but in compressed form. To use these files, you need to extract/decompress them first. If your Android device can't do this by default, then you need a dedicated app for it. There are a lot of apps that can do it, but if you need a recommendation use ZArchiver.