Hi Martin
I see you're having the usual problem with posting images and using 3rd party sites. I use SMF Gallery Pro and Post Image Upload mods on my forum, has worked well for years and you can control image sizes and file sizes, they are compressed on upload to the gallery. Members can either upload their images to the gallery first and copy/paste the BB codes in their post, or they can use Post Image Upload in the post editor and it does both for them. There are lots of settings for admin to play with.
You've probably tried it, but though I'd mention it in case you haven't.
Cheers
Mike