The easiest way is to resize an existing picture. Crop the image so there's not a lot of extra space around the image, and then resize it so the image itself is smaller. On another thread in the B&B forum, the online forum coordinator suggested making the images 100 x 100 pixels, which is an inch square. My Avatar is closer to 100 x 150 pixels because I wanted to keep the proportions. I made the narrower end 100 pixels, and let the software do the rest. I think the forum software may limit how big the picture is that gets inserted in the Avatar space.
You need to have software on your computer that allows you to alter images (unless someone knows of a site where you can do this*). I'm using Corel Paint Shop Pro Photo IX, which is similar to PhotoShop, but check what's on your computer. Rhonda
*Edited to add: Someone posted this site on the B&B forum for resizing photos into Avatars, so you might want to try resizing your photo here: http://www.shrinkpictures.com/create-avatar/