If you don't have a clue, don't try to install Imagemagick. You will need root access of the server to do it. I suggest that you ask your server owner to do it, since he will need it for other purpose anyway. Most webhost have it installed. I don't use Gallery anymore so, I don't know if it uses GD which is part of the php. If it does use GD, you don't even have to install anything.
Netpbm is very easy to install and the Gallery site has an archive for you to unpack it and upload it into somewhere simply like a CGI directory for you to start using it.
Quality wise, I like Imagemagick. But if you don't have other choices, Netpbm should be the simplest to install. You don't even need the server owners permission because it is to be installed on your own directories.
Sam