Maybe the upcoming change in the forum software, could detect and reject oversize images at post time.
Also, in the new forum, perhaps the insert picture widget in the toolbar could offer a browse your PC option, and by default insert a thumbnail. I suspect most pictures originate at the users PC.
I find almost all inline images irritating, even those within the 600x600 limits. If it were up to me I would only allow tiny pictures to be inserted full size as in --
-- everything else would be automatically thumbnailed
BR