Gravatar (globally recognized avatar) is a popular third-party profile image service. You can choose to use a Gravatar image with your Submittable profile.
If you don't already have a Gravatar account, you will need to create one at Gravatar.
If you do already have a Gravatar account, your Gravatar image is associated with a specific email address. Make sure your Gravatar account uses the email address you use for your Submittable account, and the image should just show up in Submittable.
Here is a page where you can test an email address to see if a Gravatar is set up correctly: