You'd probably start at this point after you've registered with BMC Communities, logged into your account and looking to complete your profile as suggested.
On the Communities landing page click on the 'default' avatar image on the header bar
Which takes tou to your profile page: https://communities.bmc.com/communities/people/<your_username>
Click on the 'Change photo and avatar' option on the Actions bar which takes you to your Profile and Avatar selection page
It is a 2 step process.
... by clicking on the 'Add photo option'
You have an option of selecting upto 4 profile images and choose any one to be your 'Primary photo' or profile pic.
After you comply by the image size restrictions for your profile pic it is uploaded successfully. [You can have 4 profile image options]
Choose an avatar image
You can choose from the choice of avatars (default images + mini version of your profile pic) on the list - click on one to choose it. [you can have a choice of 5 Avatars]
That is it - you've successfully updated your profile with a profile pic & an Avatar which will show up against your stuff in BMC Communities!