By ErroneousThese are the missing thumbnails for boxarts, snaps, and titles from the Libretro Github page https://github.com/libretro/libretro-thumbnails/tree/master/Sony - PlayStation 2
Just download the zip file, extract the folders and files, add to your RetroArch installation folder. It should look like:
Retroarch thumbnails Sony - PlayStation 2 Named_Boxarts Named_Snaps Named_Titles The file names for the art and what you have your game names in the Retroarch program must match exactly. You can select your game and rename the games to match the thumbnail files. The thumbnails have underscores (__)for slashes (//) intentionally as this is how Retroarch interrupts forward slashes as Windows can not have file names with forward slashes.