You can edit the avatar from the admin > users > search > find the user and edit their profile > and upload your version of the avatar.
You can download their version, photoshop it, and upload it.
If you just want to "tag" their avatar with some overlaying css without actually updating the avatar you can add some css to the right template, or through extra.css
There's a plugin that adds "OP" in the corner of the thread-poster, you could see how they've done it.