JonnyB,
If you edit the link using Database Explorer, the you are only changing the picture that appears when you click on the thumbnail picture or if there is no picture, the "Photo not available" caption. I think that the link only appears once the photo has been autopopulated by RB.
The problem with airframes.org is that it cannot match up a picture if the registration is unusual - say 264 - as there might be loads of pictures with 264 in the registration. Something unique like G-EZBB is easy for airframes.org to find.
The way I correct the thumbnail picture, or add one if it's from another site, is to save a thumbnail picture (approx 200x150) to the /Data/Photos directory. The photo names must be like this:
G-EZBB.jpg
G-EZBB,2.jpg
The ",2" is added to the name of the second photo.
EDIT:
I've just tested putting in a link to one of my photos on Flickr using Database Explorer, and it works OK. This was for an aircraft with no picture on airliners.org G-BYXO G115. First I saved a thumbnail picture to /Photos, then I added the link to the full size picture on Flickr using Database Explorer.