User / author / gravatar images
The photos come from gravatar.com. If you (or the committers in your project) have an account at gravatar, Tower gets the image for this account (account identifier is the email address).
An important thing to note is that the email of your Gravatar account must be the same as the email that the committer / author of the corresponding commit has.
So, if you committed with your git config email address set to "email@example.com" and your gravatar account has the same email address, then you should definitely see your picture.