Sorry for the delay. Yes, it's a known issue. Not really an "issue", all the images still exist, but due to a change I made to speed up the website there are approximately 350 images I have to manually create thumbnail images for. I've made a code change so all future uploaded images will have thumbnails automatically created, but I have to manually create them for everything that was previously uploaded. I started with close to 500, I'm down to 350. They'll all be visible in the next week or so.
Believe it or not, I'm still working on the remaining thumbnails. Knocking off a couple dozen a day when I can, but my job that pays the bills has kept me busy, and it's Concert Season (I was one of the lunatics that braved the Noreaster in NYC to go see Samantha play). I haven't forgotten about this, it's just proceeding slowly.