Are you installing on linux server? not sure if cpanle is available for windoze yet but if on linux you don't need the .exe, just use locate command to find mogrify, mine is the /usr/X11R6/bin/mogrify which is what you would use in photopost
I have it installed. You can do a &which mogrify& in a ssh session and it will tell you the path. I think it's /usr/local/bin
Make sure you actually have ImageMagick installed.
if you have root access log into the WHM and go down to install a perl module. Type in Image::Magick it will display a list of a cupple diffrent things... Click Image::Magick, it will install image magick as well as morgify
you can also log in as root via ssh and do run this