How about modifing the Select a Photo window that pops up when the user clicks Img so users can insert photo's stored on other web sites?
The way I would see this working is similar to the Video option:
The top of the window is the same as it now, user can select a photo from their TP photo gallery.
Below is a section to choose a photo service (Flickr, Kodak, etc - just the most popular ones) and paste in the appropriate link.
Without this option, I tried to write some instructions so we could comply with Flickr's upload policy. Seems like putting at least part of this code into the Select a Photo window would be slightly easier?