To get thumbnails to work the ROMs must be in a playlist. RetroArch has an automatic way to do this by automatically sorting your ROMs based on name and system into different playlists, one for each system. I don’t care for playlists so the method I will show will work by adding your ROMs to favourites, copying the favourites lpl file -- which is in itself a playlist -- and making a playlist out of that. Thereafter, you can make the thumbnails appear in the Favorites list too by "Add to Favorites" again via the interface.

If you are interested in the automatic way instead, see the resources linked in the "Further reading" section.

With a game running in RetroArch, open the Quick Menu with F1 and select "Add to Favorites". Each game added in this way will automatically be added to a special playlist located in `content_favorites.lpl` in your RetroArch directory.

Access your RetroArch directory; on Steam right click on RetroArch → Properties → Local Files → Browse

In your RetroArch directory, locate and copy the file called

In your RetroArch directory, access the `playlists` subfolder and paste `content_favorites.lpl` in there.

Now rename it what you want your playlist to be called, e.g. to `Playlist.lpl`.

Edit this file in your text editor of choice. Use your editor’s find & replace function to change every instance of to , where `Playlist.lpl` is whatever you chose to name your playlist file.

In your RetroArch directory, access the `thumbnails` subfolder and create a folder within it that has the same name as your playlist, without the extension. So in our example the folder should be called `Playlist`.

Inside this folder, create 3 subfolders:

1. Named_Boxarts
2. Named_Snaps
3. Named_Titles

In these folders you can add boxart, screenshots, and titlescreen screenshots respectively to your games. The images should be in `png` format and named exactly the same as the ROM. You don’t have to add all three, only what you want; by default RetroArch shows only boxarts but you can change this in the settings.

For example, to get a thumbnail for my ROM `Metal Gear (1987)(Konami)(jp)[RC-750].rom` I put an image named `Metal Gear (1987)(Konami)(jp)[RC-750].png` in `thumbnails/Playlist/Named_Boxarts`.

Some places you could get images from:

• https://retroachievements.org/
  
• https://thumbnails.libretro.com/
  
• https://gamefaqs.gamespot.com/ (may need to convert to png)
  
• Any image you find on the internet. If it is not png you can very easily convert it in any image application, for example copy it and paste it in MS Paint and save as png.

Now in RetroArch you should be able to see your playlist, and the games therein should show boxart when hovered over / selected. However, the images will still not show in the Favorites list. You might be ok with this if you are launching your games from your playlist anyway, but if you prefer to use the Favorites list like I do, read on.

In this GitHub issue[github.com], RetroArch contributor jdgleaver says this:

Originally posted by jdgleaver:

if you want thumbnails in your favourites playlist, you need to select Add to Favorites via an existing playlist entry that already has working thumbnails.

and, indeed, if in RetroArch you select your playlist, select a game on there and click "Add to Favorites", the old entry for this game in the Favorites list will be replaced with one that has the thumbnails.

Why not modify `content_favorites.lpl` directly, you ask? Because this file is generated automatically, and your changes will be reverted every time you add or remove a game from the Favorites list. If you add it as stated, it will keep the thumbnails forevermore.

• Official documentation on this topic: https://docs.libretro.com/guides/roms-playlists-thumbnails/
  
• GitHub discussion about making thumbnails appear in Favorites list https://github.com/libretro/RetroArch/issues/9751
  
• this video is also excellent: https://www.youtube.com/watch?v=JRd21tO8FyM