OK - what you need is the following line to be configured correctly;
This will typically be just this on Linux;
This depends on convert being in your path. You can test to see if this is correct by logging in with SSH and simply typing;
on the command line - you should get a help page telling you how to use it.
If not you need the full path to the convert binary (you'll need to ask your host that question). A common place to put it is '/usr/local/bin/' in which case you'd configure site.ini like;
One other thing you might be able to do, if all else fails and your host won't install it is download the Linux binary from ftp://ftp.nluug.nl/pub/ImageMagick/binaries/
and see if you can find convert (I haven't tried this). You could then upload this to your own directory on the server in point to it in site.ini e.g.