With great difficulty. Alas I've never figured out how to do it, other than to put one on my profile picture, and I daren't change it coz can't remember how to!!!
No doubt there is someone on here that has the knowledge and could help you............ alternatively ask to borrow someone's six-year-old kiddie for a few hours coz they're all so shamingly computer-literate these days.
I have not managed it, but there have been posts about this before. From what I recollect you need to have a photobucket account, upload photos to there, get a url (?) number for that photo and add that to your post.