Each user can have an avatar icon configured, in addition to being able to set up an icon for the organization itself. These avatars are used in various places throughout the system to provide a visual context in presentation, like in the activity feeds.
To set your personal profile icon, go to the Settings, then select the Profile tab. You will find a section that allows you to upload an image to use as your avatar.
After the image is uploaded, you should now see your icon showing up in places throughout the Fulcrum system.
See also how to add an icon for your organization.