All the users who are invited/added to the application are assigned a default profile picture. The profile pictures is available on the bottom right corner of the Pipeline page and can be used to assign leads to users.

To Change your profile picture, click on settings from the top menu, 

Click on Change Profile Picture, you can click on pick photo to add your image from various sources or drop a photo from your computer into the box.

The uploaded file will replace your default profile picture as your new profile picture.