Image changes are a little slow right now because of the DNS change with our CDN.
Basically, we all upload the images to our TBN server, and then when we access it, our CDN checks to see if 1) it has the image and 2) if the image on our server is the same as the one it has. If those are both true, it sends you the image it has (which, conceivably, should be faster than our server) but if it's not the same, it will request the new image and then show you that.
But it doesn't check EVERY time. Because most of the time, avatar images don't change, neither do the images people attach to their posts. So maybe it checks once every hour. Or maybe four times a day. Sometimes your own ISP does the SAME THING (sending you cached images to save their own bandwidth and speed up your access) but maybe your ISP only checks for updates every 2 hours. Depending on when people update their images, you might get a new image RIGHT AWAY, or maybe 2 hours (or tomorrow!)
The Internet works, but we definitely all do a LOT more on it than we thought we would 20-30 years ago!
tldr: If you change an image and it's not immediately updated across all your devices, just give it 24-48 hours. If it's still an issue, let us know and we'll help try to figure it out!