Go to your profile page, which is here:
https://www.suicideforum.com/community/members/lifeisblah.53494/
Hover your mouse over your profile pic, and the words "Edit Avatar" will appear. Click on it and you'll be able to upload a new pic, whatever pic you want. That's for pc. If you primarily use your phone, then I'm guessing you'd go to your profile, that link I provided, and tap on your profile pic.
Maybe phone users could confirm that.