Welcome to Bass Resource. Enjoy reading & contributing in the forums.
This is from the FAQ section:
I can't post a picture. It says the size is too large.
You need to use a photo editing program like Adobe Photoshop, or whatever your computer comes with, and go to resize image. You may want to crop it for your avatar. Once you do that you can post any picture.
You can also use Pic Resize (http://www.picresize.com/) You just need to locate the picture on your PC, and it's simple from there.
For mobile devices, get the app Image Shrink Lite for Android and iOS.