Generally, the path is /usr/bin/convert and /usr/bin/mogrify however these can vary host to host.. Also, it's required that if using imagemagick that you be running version 6.
As for the issue, as the webserver creates the directories for each forum, it's possible that (a misconfiured server) created directories belong to apache vs your user and they are not writeable.
What you should do, login to your FTP server, and check out to be sure that the gallery/*/forumnumber folders are all chmodded 777 and that you can upload to them (try uploading a dummy file); if you cannot upload (permission error) you'll have to have your webhost chown the directory to your webserer user.
