Entry tags:
Rambaldi symbol userhead

Edit (February 15, 2017): Now also with instructions for Dreamwidth.
CSS
Go to Custom CSS and insert the code.
user/comm
.ljuser img
is the general code which changes the images of both, user and community.
If you want the community image to be different from the user image, use
.ljuser img[src*="userinfo"]
or .ljuser img[src*="community.gif"]
For Dreamwidth it's
.ljuser img[src*="user.png"]
or .ljuser img[src*="community.png"]
Where will the userhead appear?
It's not like the userheads LJ sells that will show in front of only your username, whatever journal you post in.
The userhead will appear in front of every community and/or username (if it's both or just one of them, depends on which code you use [see the table cell above]) where your CSS reaches. You will see it on your journal, but not on your profile and - if you disabled customized comment pages - not on comment pages. You will see see it on other journals if you view them in your style (put ?style=mine
at the end of the URL).
Others will only see it when they go to your journal.
ETA: How to make your userheads work correctly