If you go to your account settings, you will see you cannot simply upload a picture. The picture must already be hosted somewhere, and you then must copy the URL and paste it in the designated area in your account settings.
To do this, making a post in #introduceyourself is a great way!
You can make a post, upload your desired picture in a post, and now you should see a URL portion of the photo you just uploaded, which you can copy into your account settings to use as your profile picture.
Don't hesitate to reach out if you have any questions. (Go Seahawks!)