From CUNY Academic Commons
A gravatar is a digital image that follows you from online site to site, appearing next to your name, identifyng your posts on blogs and web forums. Gravatar is an abbreviation for “Globally Recognized Avatar.” Free gravatars are provided by Gravatar.com, where you can get one that’s already been created or you can create your own. When you register for a gravatar, your account is based on the e-mail address you provide. From then on, whenever you register in most online sites (especially blogging sites) using that e-mail address, your gravatar loads immediately.
If you want to create your own gravatar on Gravator.com, you can load any jpeg image. The maximum image size is no larger than 500 pixels in length OR width. If you upload an image that’s larger or smaller than this, Gravatar automatically rescales it (and does an excellent job of keeping the image’s sharpness). When the gravatar displays, it is always square and always displayed at 80 by 80 pixels.