This is nice, big +Rep

Will try it out ! Ty for the idea

In the code you write that you don't know how to work around addons that don't match the mapname:
- build a filetable of all the addons by doing file.Find with the addon.title from engine.GetAddons() as the scope
- look through each file table and find the one with the highest wsid
- now that you know the wsid, you already know how to get the image

here's some code I happened to be working on coincidentally github gist [gist.github.com] that you might find useful

Thanks and good idea! Sadly there is no way to automate that process since gmod only allows to write into the data folder

This works pretty well, I found it more useful to symlink the garrysmod/data/map_thumbnails/maps/thumb folder to garrysmod/maps, that way I only need to restart and not manually move/copy the thumbnails.