Here are my 2 cents, since there are lots of questions & suggestions floating around:
I too like the idea of having to post an image in order to start a thread. It could be almost exactly the same as it is right now. The main page could still look identical to the current one -- a place to post general 'shouts' in reverse-chronological order, as we clearly enjoy doing. And at the right, the 5 image thumbnails, and maybe just beneath each image is the thread title, if the originator chose to create a thread along with the file.
Then, if you click on any of the 5 image thumbnails, instead of getting just the image in the browser, you would see the image at the right (possibly occupying the same space that the 5 thumbnails do on the main page) and the thread in chronological order at the left.
When you replace the image, you also delete whatever thread went along with the previous image. When you upload a new image, there's also a "start thread" box below so you can tag the image with a thread-starting piece of text, and give it the title which appears below the thumbnail on the main page.
Final suggestion: Have thread pages be a different color than the main page, so they look significantly different.