Thread:Project:Support desk/error creating thumbnail unable to open permission denied

I'm getting the error:

Error creating thumbnail: convert: unable to open image `/home/user/public_html/wiki/images/thumb/a number/another number/filename.jpg/120px-filename.jpg': Permission denied.

This was previously asked here:

http://www.mediawiki.org/wiki/Thread:Project:Support_desk/Error_creating_thumbnail:_convert.exe:_unable_to_open_image_(folders)/public_html/deadrisingwiki.com/public/w/images/thumb/d/d8/CaseWest_04.jpg/120px-CaseWest_04.jpg_Permission_denied_@_blob.c/OpenBlob/2480.

The solution to that problem was to correct the chown the user from "root" to the mediawiki user. In my case, everything is already set to the mediawiki user, with chmod 755. So, what's wrong, and how do I fix it?