*Drumroll*
Found it In 'My account' I had image default set to 'upload'. By changing that, I now get the screen you mention. I can't believe it's taken me two years of intermittent fiddling to finally get it sorted.
After reading your reply, I took a while watching the tutorial, then scanning every tab until I saw something, anything, that might change my settings. So thanks, Pete, I appreciate the reply. Even if it didn't cure it, you confirmed what I'd always thought; that I should be able to view auctiva-hosted images to load onto new listings.
Many thanks.
Baff