Hi Bill,
Once photos are posted and go to the big Hoop.la server you have no more control over them. You can search for your old threads, but not specifically for the photos they contain.
For example, if you want to re-post a picture later in another thread, you can't retrieve it from a previous thread and paste it into the new one. You have to find the photo again on your computer and upload it again.
I started trying to keep track of which ones I posted like you suggested, but gave up. Luckily, most forumites don't seem to remember any more than I do which ones are repeats. I've never gotten any complaints.
We like to think our photos are memorable, I guess, but actually....!!!
Jim