This was posted as a fix - it worked for me - I simply found those two lines in the file and changed them to be exactly as he shows below:
I found a simple fix for the includer I use.
When you have installed 5.0 and up you have to rebuild your thumbnail database first.
Then you have to change only two links since the thumbnails are placed in the 'thumbs' folder.
My links look like:
$filepath = "$data_dir"."$thecat/thumbs/$photo_name.$theext";
$rel_path = "$rel_dir"."$thecat/thumbs/$photo_name.$theext";
See samples:
http://www.xboxusersgroup.com (thumbincluder build into vBPortal)
http://www.xboxusersgroup.com/forums/games.php (thumbnailer build into a game review hack)
http://www.xboxusersgroup.com/gallery/index.php (Our Gallery).