One big caveat to adding an avatar is that it must fall within a specific size (dimension) range. That range is currently set from 75x75px to 150x150px. Knowing that most people don't have a quick way to take a picture of, let's say, their Jeep, and reduce the size down to what the forum requires. So, I found a website that does it for you!
https://www.shrinkpictures.com/create-avatar/
Go there, upload your picture, choose the size you want (somewhere in the aforementioned range, although it will probably look best if you make it at least 125x125px) and click "Resize". A few seconds later, you'll be taken to another page where your image will have been reduced to a tiny little square. Pick the one you want, right click it and download it, or use the link below the image to download it. Either way, you'll have to download and save it. You can't remotely link back to their site, as they purge all their stuff periodically.
Not too bad right? Give it a try and let me know if you need help!