If you're on a laptop/desktop computer, click on your username in the top right corner of the website, and a drop-down menu will appear. Click on your (currently blank) avatar:
If you're on a mobile device, tap the hamburger menu (the three lines in the top left corner):
This will open the main menu for navigating the website. Next, tap on the person icon as shown below:
Here you'll see a list of links where you can see various information about your account. Tap on the account details link to continue.
If you're still having trouble, here's a direct link to the page:
Now that we're on the right page of your account, you can upload any image you like as your profile picture/avatar by clicking/tapping on the avatar image:
A window will appear where you can choose an image to represent yourself in the StP community. Once chosen, it will automatically appear anywhere you've posted content.