At last I've found other radarbox owners with the same photo not available problem. I cannot solve the issue either. I have the latest 2008 download and the problem appears to have arrived with that download. G4MHQ
If the photos are not downloading, I can assure you that it is nothing to do with V1.5 which I have been using since it was released.
However, there does seem to be an issue with the photo download at the moment, but as I don't quite know how this works, I can't suggest where the problem might be. I suspect that RB gets the thumbnail photos via the AirNav server rather than direct from airliners.net, so in that case it is an issue for AirNav to look into.
As a temporary, although time consuming fix, it is possible to manually download the thumbnails and save them in the /Data/Photos directory. I do this anyway for pictures that are not on airliners.net, but are available elsewhere. I use a thumbnail width of 300 pixels. Remember that naming the photos is critical and the format is:
G-AAZZ.jpg - for the first thumbnail
G-AAZZ,2.jpg - for the second thumbnail
This is fairly impractical for all the thumbnails (I have over 16,000 jpg's now) but if you really need a thumbnail for a particular aircraft, then this is the answer until AirNav sort out the server - if that is the problem.