As a site administrator you can add a default avatar for commenters who don't have their own yet. This adds a nice touch to the Disqus embed to make it feel like a part of the community.
To override the default Disqus commenter avatar, go to your Disqus admin, click the Settings page and then the Advanced sub-tab. The option will be under Default commenter avatar.
Keep it simple. Use an image that anyone in the community can associate with, and that doesn't distract from their comment.
Avoid showing anything that represents a specific person or brand other than your own. Users reading through the comments might confuse the default avatar with something that the commenter chose.
Don't put a picture of yourself. To change your own avatar, see Updating Account Settings
If you prefer the original default avatar, you may download the one below and upload it in your Disqus admin again using the same instructions above.