I have thumbnails for stock VSTs and one for serum.My others ie: arturia pigments,rob papen predator 3 etc not in vst Steinberg thumbnail folder,it’s empty.
Word of warning
Thumbnails are stored in your RAM once you open MediaBay in your project. This can eat up a fair amount of your RAM if you’ve got lots of them in high resolution. If your RAM is limited then you might want to reduce your image resolution.
Here’s more info about how, where and when