If you use something like Dropbox that syncs files on your computer, you could save the JPEG to the same (publicly linkable) location with the same filename (i.e., overwriting it) and it would at least update the image without you having to update the forum post that links to it.
If you use something like Dropbox that syncs files on your computer, you could save the JPEG to the same (publicly linkable) location with the same filename (i.e., overwriting it) and it would at least update the image without you having to update the forum post that links to it.