So i managed to get a dump of the NSO NES app and a lot of the formats are just standard PNG's, WAV's, lua scripts, .nes roms etc etc.
But it seems like the game box art is stored in the .XTX format(which tools exist for), However they have an extra .Z extension, Opening it in a hex editor shows that the files are seemingly compressed, and im willing to bet the .Z means zlib or zstd.
I ran the file through TriD(http://mark0.net/onlinetrid.html
And it showed a 100% match to zlib compressed data.
It could be wrong still but i thought i'd mention that.
Here's one of these files.https://cdn.discordapp.com/attachments/419711036837330956/492673879484137482/CLV-P-NAACE.xtx.z
If my research was correct this should be the Super Mario Bros 3 cover art.
This is the only format that currently can't be tinkered with at the moment. Everything else is bog standard stuff.