This is not about being a member of the other forum (BTT). Many members of BTT practically struggle here to understand how things work.
For the avatar, I think the same dimension that works for BTT can work here. Just go to your BTT profile, as if you want to upload avatar.
Just checked it : 120px wide, 80px tall and not more than 100kb
I would also like to suggest this: Signature campaigns have the most suitable avatars for the forum, and we can refer to their sizes, usually 120x80 px. In case members want to try other sizes, they will need to test or refer to instructions on the maximum size and capacity for images.
Sometimes errors also occur when members use URL links to images, and forum cannot easily read information from those links. In this case, members should download the images and then upload them to the forum. I have also encountered this situation: I can see my avatar, but others cannot see it
