Without using SSI there are two ways to do this.
- Have the script output an image
- Requires the user to have a browser supporting images (and for them to be enabled).
Disallow access to the real files and force users to access only the script. Use the query string to decide which file to load.
- Really, really ugly URIs
- You have to change all your links
- Search engines tend to dislike sites consisting of one with with different query strings.
Really, the best solution is probably going to be finding a better service provider.