Since I bought Mafia Classic, the tile for it in Galaxy was blank.
So I finally found out that the file "bd46a9e35faf6478d91248820404d5ade09b855cc86a01d25a44201191a9cc92_glx_vertical_cover.webp" was corrupted. For reference it's located in "C:\ProgramData\GOG.com\Galaxy\webcache\<UID I think>\gog\1595659240"

I could replace it with this picture, nice but I suppose it's not the same as the originally intended one. It's not a huge problem of course but still a bit of an annoyance.

So can anyone send me his picture so I get the real gog one ?

(GOG Galaxy use both jpg and webp as file resolution and they are the same absolutely identical pictures. On API links you can replace webp to jpg or jpg to webp and another file opened/downloaded or just rename already ownloaded file)

........
Try remove files there and verify game in Galaxy client at least twice in a row (Galaxy no more report results of verification process).
As Admin of course.

Hmm... There is User related folder with settings and even saves for some games. But in User AppData as a i remember. Maybe im forgot that. Be careful erasing GOG folders.

Thats related to Galaxy versions, Cloud saves. And more important - GOG versions (offline) sometimes store saves there too (as UserID 0).

.......
Such sites first upload official storefront assets. SteamCDR and GOG DB. Look here for its galaxy-pictures and box-cover
http://www.gogdb.org/products?search=mafia

This DB site unfortunately kinda abandon. It did not show all pictures. Logo and some background types pics not shown and available at API links only. But what you look for - box art - are there.

(there was updated fork of GOGDb which is only partially implemented in main site - closed one have all pictures listed as well as more correct in some information/history listing).

The gogdb site hinted me to the right picture, and the correct one is also on steamgrid.com, so I could use this one instead.
Thanks !