When you save a photo in a thread, it is actually saved to a separate location on the server. An 'active' link to that photo location is stored within the thread. When a viewer opens the thread, the photo is also opened and displayed because of the active link. Well, at least this is the way that I understand it works. So, if you remove the copy that is in your upload manager, you will also be removing it from the thread where you posted it.
The advantage of this is that for many of us, we end up responding to the same question in multiple threads over a period of time. When we respond, we can attach a photo that has already been uploaded and stored on the server. Thus the same photo can be used in multiple threads without being loaded onto the server more than once. This saves quite a bit of disk space for us on the server, plus it makes it easier to refer to a given picture without having to find it in an obscure thread someplace. It also helps to prevent degradation of system performance on the server, and that is a good thing!
