I've just tested it myself, and I understand your issue. I tried to change the background to an image, I get the little pop up box, I click 'use image' and go to choose an image and I get that little dialogue box with the spiel regarind 'using asset picker...' blah blah blah. You're right, the 'okay' button is hidden beneath the border of the browser, or in my case, my windows bar down the bottom (don't remember what it's called) and I can't leave the screen either. Thankfully I opened it in a new tab from this one. I honestly don't know how to fix it, perhaps one of the admins has some insight for us?

Just as an additional note, thanks for bringing this up, because prior to this moment, I didn't even know we could customise our profiles like that