One of three ways.
1. Log in and choose "Change Profile Photo" from the member admin area.
2. Log in and go to your profile. Click on Profile under your picture and then Change Profile Photo under that.
3. Log in and go to your profile. Put your mouse over your profile picture and click Change Photo.
You will be able to upload and crop your avatar (picture). Cropping your image is simple. Once the image is uploaded, it will be displayed with a dotted line box in front of it. Drag the corners of the box to change the size. Drag the center of the box to re-position it. The smaller image on the right will display what your avatar will look like. When the box is where you want it, click the Crop button. (You may need to put the cursor where you want the upper left corner and hold the mouse button while dragging the mouse to select the section you want.)