How do I find out from within an HttpModule what domain made a particular request?
Say I only want to allow site1.com and site2.com to use images from my server, how do I check that it is them making the request?
There's no way to do this in every case. Consider that UrlReferrer may not be set. Also, consider that you could be called by a client that does not have a DNS address.
Instead, you should consider configuring IIS to authenticate using user certificates. If you've only got a small number of sites calling you, generate a certificate, register it with IIS and map to the user you want, then give the certificates to those two machines to be installed on them.