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On the top right of the browser window click on your user name and then click on My Profile:
As you hover over your current avatar a "Change" button will appear, click it:
From there you will have the option to upload an image from your computer or upload from a URL.
The description says that at least 200x200 is recommended, but in reality 200x200 is the maximum dimension you can upload. The file size for the upload is limited to 90 KB.
You then have the option to crop the uploaded image if that is what you want to do. The avatar shown in posts is 100x100 and the avatar shown in your profile is 150x150. My recommendation is that you upload an avatar that is at least 100x100.
Avatars which are smaller, or not square, will be stretched to 100x100, or till the first dimension is 100 and the other is whatever it is.
