imgur with a registered account so you can enable "Upload images in high quality" to disable compression. It's been around since 2009 so it should be reliable too
The maximum filesize the forum tool here takes is 1.2mb. Those png files I uploaded earlier were roughly 500kb each and they still got resized.
I don't see any image upload guideline on the "forum rules & regulations" page.
Seems like the resize was just a limitation on the Xenforo™ software, but a something like that needs be stated on the regulations page.
It is in one of our boards but I posted it quite a while ago. We had a lot of full res pictures after the PC launch I believe that we’re murdering bandwidth. This isn’t bad though.