I'm trying to figure out where custom game media is saved. One of my games (Rogue Heroes: Ruins of Tasos) was missing the tile artwork and I just went to grab it off of steam when I learned about a site called steamgriddb.com where people upload custom artwork for games.

I got the idea of making a small script that would pull random artwork from steamgriddb.com and assign it to the proper games in GOG Galaxy to add some variety in my library.

So I guess my question is kinda 2 fold here:

1. Where does GOG Galaxy save the custom artwork provided by the user?
2. Would simply placing the files in the save location be enough for GOG Galaxy to use that artwork or would I need to tell it to specifically use custom artwork?

it depends on a few things like is the game Cyperpunk [has its own special place], Gog store or a 3rd linked

yes you can simply link images and on a local network that will be ok for a while but it is not the way Galaxy is designed and will not sync

my recomendation;
1, look at the images I uploaded to see the correct way to swap images for your games [1]

2, after Galaxy does a sync, it will convert your image from jpg, Png or whatever and save \ store to C:\ProgramData\GOG.com\Galaxy\webcache\... and saved as .webp format

p.s, size matters for images both how well they look and how long it takes to sync them

Thank you!
I know that, that is the intended way, though it would be cumbersome to do this on a ~2000 game library or even just for the 80 games I have installed.

I wanted to automate the process to make it easier, and steamgriddb does have images in the right sizes.
The folder structure does seem a bit interesting, I'm guessing it's where GoG pulls the images from?

the 80 games you have installed + any you have already swapped out backgrounds for in your library will be stored in that webcache so yes you could use a batch to swap out the images with others, rename the new files to have the same dimensions and name and Galaxy will show them locally but they will not sync because [as far as Galaxy knows] they are the same file witch has not changed

personally it sounds like a lot of pointless F#^ing about for 2000 games you probably don't play that much but it's your time so make your own mind up