I can not tell you what your mogrify path is you might need to get your host to tell you whether they have it installed on the server.
Before you attempt the test edit we do suggest you also clear the uploads directory on your photopost server. You ftp to the server and go to your Photopost directory. You click to get into the uploads directory and then simply delete all the numeric directories underneath the uploads directory.
You can then test if the edit worked. Your uploads directories for certain users could have too many files that no edit is going to work until you clear those directories