automatically embed image in all requests

hi all,
could someone give me some ideas on how to automatically embed an image
into all requests to my apach 2.x server? i have ~20 virtual domains
that i want to embed the image into all pages, without having to edit
them all to includ some sort of ssi, in order to do tracking. if i
could somehow specify the image by virtual domain, that'd be even
better.

thanks!

jeff