I gotta say I'm having exactly this problem on a photoplog lite install that I'm evaluating at
www.classicrallycars.com/photoplog
Thumbs will load OK, then on next load will be replaced by the alternative "image" tag. Following the "image" tag can give a permission denied. Then on next load of page that thumb maybe OK and some others will vanish. Then sometimes they're all OK.
I'm experienced with php and installs etc. Permissions are correct as far as I can see. safe mode is off.
This is under vb3.6.5 (as you can see on the site).
Any suggestions?
Quite like the script, but can't purchase until I can see this resolved.
Cheers