Go to your control panel, and use the link on the left to Edit Avatar. On that screen you will see the specifications as to size and file types allowed. Create an image you like that meets those specs. Since it looks like you have a website, you could upload it to that site. Then, go back to the edit avatar screen and enter the address on the Link to field. I'd think it might be something like:
http://www.davisbeantrees.com/myavatar.jpg
though that would very depending on what you called the image and where you loaded it on your website.