iGal needs Perl to run and either cjpeg/djpeg/pnmscale (from libjpeg) or ImageMagick to generate your thumbnails. These come standard with most Linux distributions today.

igal is a command line tool, so if you take that approach you will probably need to adapt it to your own code. Looking at the code, igal calls either imagemagick or cjpeg using system, so if you adapt something from this code you'll have to be very careful about validating filenames etc, to avoid attackers being able to run arbitrary code on your webserver.

As far as installing modules without direct access to the server goes, have a look at this node. BUT, both GD & Image::Magick act as wrappers around existing libraries. If either of these libraries are installed on the ISPs web server, you could be in luck.

With the 'imageweb.info' approach, I suppose you could upload the file to your own script, then use LWP to upload the file from your script to imageweb.info & screen scrape the returned URL. Quite messy, and probably quite slow, but possible. If the client browser uploads the file direct to imageweb.info as you initially suggest, you won't have a connection from your script to the returned page to be able to grab the returned URL. The other problem is if imageweb.info changed their upload/response page drastically your script would stop working.

There is also this site, which returns a scaled graphic rather than a URL.