I just now finally figure what my problem was and fixed it... before uploading images (dragging and dropping them in), I made sure there were no parenthesis, spaces, brackets or any other wierd characters in the file names. So I replaced all spaces and parenthesis with underscores (like this "_") and then re-added them, then poof, all my thumbnails showed up finally! :D

When I first tried to fix it, I reduced my image sizes to 800x480 or so... but that didn't actually seem to be the difference. It was the file names that actually caused it for me.