What is a gravatar? As per the site, "A gravatar, or globally recognized avatar, is quite simply an image that follows you from site to site appearing beside your name when you do things".
As per Wikipedia, "A Gravatar avatar can be up to 512 pixels wide, is always square and is displayed at 80 by 80 pixels by default. If the uploaded avatar is larger or smaller, the avatar is scaled appropriately".
I wanted to add one to Stack Overflow.
So I searched Google Images for a small image and saved it to my hard drive as a .jpg.
Then I used the ubiquitous Paint to open it. Under "Image / Attributes", it showed as:
Width = 60, Height = 40.
To keep it square, I needed to make the width 40. Simple maths shows that I needed it to be reduced by a third. So "Image / Stretch/Skew" and type 67 (i.e. 67 %) into the "Horizontal" box for "Stretch".
Another "Image / Attributes" showed it was pretty much as required. Save on the hard drive.
So logon to Gravatar. "My Account / Manage" Click "Add a new image". Browse to where the file was saved, select the file, crop as appropriate (I just dragged the box to fit the image), select the content type (hopefully it's "G"!) and you're done.
Now any site that uses gravatars will display your new image.