This is the procedure if you are on a computer.
At the top right of the page, click on your current avatar.
On the page that pops up, click 'my profile'.
At the top right of that page, click on 'profile images'
You can upload an avatar image or a header image, or both, but one at a time.
Study the details of the parameters for the image you want to upload.
Make sure the image you plan to use meets those parameters.
Continue to follow the instructions to upload your image.
The browse button will allow you to navigate to the location of your image.
Select it and then upload it.