If you can't install modules, or they won't install modules, you might want to check if the netpbm tools are installed, if the libjpeg tools are also installed, you'll be able read and resize most image formats you're likely to need. These tools are pretty common, otherwise the imagemagick tools might be installed as someone else mentioned.

In general though, if your host won't install the tools you need to host with them, then it's time to find another host.