Installation and images on separate servers

I have 2 separate web spaces from 2 ISPs. One is fast and reliable and has better services, and I have my main site there, which uses mambo and SMF (bridged).

On the other webspace, which is much larger, I have the large downloads, and the photo gallery with thousands of photos. (Its a music related site). I simply link to from the main site when users go to the Photos section, and it opens up in a new page.

The main trouble with this is they have to have 2 different accounts.

My ideal solution would be to be able to keep the images themselves on the larger webspace but install a gallery on the main site (and integrate it with Mambo/SMF so that users only need to log in once.

So when users upload a photo, they would really be uploading to the other server.

How could I go about this if it is at all possible?