First of all, an Avatar is the image that appears under your username inside your post on the message boards. The Signature image appears directly under your post on the message boards. Both images are optional. Avatars and signature images are only viewable if you are logged on to the site.
These images should not be a distraction to a user and should be safe for viewing at work. Images with excessive animation are not allowed.
Images should be no more than 30 Kbytes in size. Anything larger than that can cause threads to load up slowly.
Avatars should be no more than 150 pixels in height and 150 pixels in width. Signature images should be no more than 150 pixels in height and 400 pixels in width.
To view an image's pixel size, right-click on the images and go to Properties. The dimensions are shown as width by height.
If you need assistance with these images, please post on the Help Board.