Colin,
Not that it matters much, but I think that the forum platform actually does automatic size reduction of too large photos, at least within some limits.
My photos are mainly taken with a Leica D-LUX 4, and come out of the camera as JPGs roughly 3750×2500 pixels in size and weighing in at around 3 to 4 MB each.
Those images that I select for use on this forum (and on another, Swedish one, where I also participate), I always edit in Photoshop, both to give them more meaningful file names, and to re-size them. After this process, each photo is 1500×1001 pixels in size and, at 180 dpi, weighs around 1 MB, which is pretty much the optimum size for use on that other forum.
This is thus the size of the files that I upload to this forum, too; however, during the upload they are automagically saved to the MB forum servers at 1024×683 pixels and 96 dpi and weighs in the low hundreds of kilobytes.
I have never tried to upload any full size photos to the forum, so don't know if there are any upper levels above which the system would refuse the files rather than re-size them, but at least from 1500×1001 to 1024×683, it does happen!
Mattias