Regular HTTP image:<br>
<img src="http://www.gravatar.com/avatar/f52a1d3d8c39868fc92cf70828821635?s=80">
<br><br>
Regular HTTPS image:<br>
<img src="https://secure.gravatar.com/avatar/f52a1d3d8c39868fc92cf70828821635?s=80">
<br><br>
Using avatar.php:<br>
<img src="http://www.gravatar.com/avatar.php?gravatar_id=f52a1d3d8c39868fc92cf70828821635&size=80">