Look at the top right area of the window where it says Welcome, [your name], Notifications, My Profile, Settings, & Log Out.
Click on Settings.
On the next screen that comes up in the left hand margin (you might have to scroll down a bit) you'll find Edit Profile Picture. Click on that. Then you will have an option to load one up from the internet (Option 1) or you may load it from your computer (Option 2). If you click on Browse you will be looking through your personal files on your computer and you will be able to navigate to a photo of your choice. Select it.
I hope this works for you. Welcome to the forum.
—Jim