I've never had an Apple device in my hands, but they should have the ability to resize and post pictures directly to the forum. I do have an Android tablet. It is a pretty crude and useless device compared to a computer, but it has the ability to resize pictures and if it had some kind of security that I could trust it to hold any of my pictures, it should be easy to post them directly to the forum.
Any device that uses a cloud somewhere for storage of files isn't a device I would ever trust. Apple has been hacked numerous times and leaked private information. You are much better off using a computer and shying away from any cloud storage. Keep your personal files personal and backed up at home if you care about them.