Yes, I can see how this would be problematic on a forum like mine where we have over 2,000 categories of photos and more than 200,000 images ... trying to select a specific image navigating the gallery via a popup would become very clumsy I feel.
If you were just showing a list of titles, it might not be so bad - but you'd also need to show thumbnails, which makes the interface larger and more demanding.
I'm not sure it would actually be any quicker than just opening the gallery in a new window and browsing the galleries to find images and copy-paste the BBCode from there ?
I'd love it if someone could come up with an elegant solution though.
How about a popup search interface where you could start typing some keywords and using type-ahead it shows you thumbnails of images so you can narrow it down quickly to a specific photo you're looking for and insert into a post - that might be workable?
If you don't know what you're actually looking for to know the search terms, then you might just have to go browse the gallery by hand, but if you do know eg "elephant san diego zoo", then I think some kind of search popup for inserting images into posts might work ??
Just a thought.