Adding your profile photo is a key step to take prior to sending out your first proposal in Valve as it will pull through to any proposal you send.
In order to edit your profile photo:
Click on the user icon in the top right hand corner of your screen.
Click Profile
You will now see your Profile Details.
Click Choose File at the top of your screen
Choose your Profile Photo in your file explorer or Finder window.
Click Save at the bottom of your screen.