You're allowed 1 'profile picture' to be displayed in your Profile. Go to 'My Home', and click 'Edit Profile Picture' under 'Settings & Options'. You may either (a) enter a URL to a remotely-hosted picture file or (b) upload a picture file from your computer. The maximum size for a profile picture is 300 X 300 pixels or 97.7 Kb (whichever is smaller).
For your avatar (little picture under your name, like my laughing skull...) click on 'Edit Avatar' under 'Settings & Options'. In addition to the same options mentioned above, you can select from among 10 'stock' avatar pics (or not have an avatar at all, if you choose so ...). The maximum size for an avatar is 100 X 100 pixels or 19.5 Kb, whichever is smaller.