You can select GD all you want in the Photopost config but you would still need it installed on the server.
Since one of the requirements of the program is to have either ImageMagick or GD you would need to find out from your host which you have.
In your admin section of Classifieds should be a phpinfo link
If you do not have GD installed you would need to try finding out where mogrify is on the server.
If you have neither processor installed you would need to have your host install one or if you have shell access try installing a non-root install of Imagemagick.