To get started, make sure the picture you want to set as your profile picture is on the computer you're setting the picture from, then login to Moodle.
Click the arrow next to your name, in the upper right corner of the window, and select "Profile".
Select "Edit profile" on the next page.
On the next page, you'll need to scroll down, until you see "User picture". Then, you can either drag and drop your image onto "You can drag and drop files here to add them." or click on the green "Files" button which will open a Finder window (macOS) or an Explorer windows (Windows) so you can navigate to where your image is located.
Once your picture has been uploaded, scroll to the bottom of the page and click "Update profile".