You cannot change your profile picture. This is pulled automatically from your LinkedIn profile. If you would like to change your picture, you can change it on LinkedIn, and it will automatically be updated the next time you sign into TALK. You may need to sign out and sign back in for the changes to take effect.
To sign out, click your profile picture in the upper righthand corner of the screen and click "Sign Out" from the dropdown. Then click the "Log In" button in the upper righthand corner and use LinkedIn to sign in.