That is more than likely why it does this - users can choose to display more than 10 posts at once, and if every avatar is the size of wallpaper it’s going to churn.
There’s a much larger limit on files attached to messages. When users attach a large image and display it inline, I know sometimes there’s a short delay before it shows up.
The good side is you only need to resize your avatar once (unless you’re changing it like we’re Instagram!). I’m pretty sure Preview on Mac and Paint on Windows will let you do this. For anyone a little confused about the process, the image software I use is free (ad-supported) online:
To resize the image, select “Open Image From Your Computer” (crop as necessary using the crop tool which is usually where it will start you).
Then go to “Image>Image Size” and you can type the exact number of pixels in, constraining properties will keep the image the same ratio with regard to width and height, or you can uncheck that which may stretch your image one way or the other.
To use an image as your avatar on this forum, it must be no more than 100 x 100 pixels, and file size no more than 11.72 KB.
Hit Okay, then Save the file to your computer and upload it into your profile under User Control Panel and the Profile Tab > Edit Avatar.