In what folder does galaxy 2 store its cover images?

I don't think we know that, it's probably just a temp one. (we believe those are saved on gog's cloud)

However! For non-changed images, they pull those from a gaming database (mostly the same Steam is trying to do now with their modified library). So, they are probably not saved locally, not even temp.

EDIT: If you know some database stuff and programming, I can probably teach you how to access some specific stuff, but there's no easier way afaik.

I may get ban for this for here goes... for example, background for 'Gorky17':

https://images.gog.com/75742662021ac27634ec96afc43ff7316219c0694160cc19a9f480712068ea30.jpg

But these links probably change over time, as the local database gets updated and uploaded.

I changed all of my images,so there is no way as of now?

I think that's just as likely for me to NOT find them locally, actually, as they too would likely be sent online/cloud. I'll keep digging (for another unrelated personal project of mine), but I'd suggest you keep track yourself in the future (creating your your 'custom art' folder), that's what I did anyway. :) Sorry for not being more helpful.