Its simple really, just follow these steps:
(I'm assuming you have already signed up at scirra.com and and you are now on the gravatar website)
1. Type the same email address you use on scirra.com (very important) into the text box and click on "Get Your Avatar"
2. click on the blue button that says "OK - email@example.com is in my contacts"
3. You will be sent an email from gravatar.com to confirm your email address
4. Check your email to get at the email from gravatar.com (this step is important) and click on the link which will take you back to the gravatar website.
5. type in a password. hit "signup"
6. if all goes well you'll be taken to where you'll put in details about yourself(not necessary) and upload an image of your choice.
7. Once done, just logout from gravatar.com and refresh/reload your scirra.com webpage
The image you uploaded to scirra.com now should show up.
Hope this works for ya...