You can upload a photo to an individual person's profile on your tree by first clicking on their name and then the 'profile' link.
From here, you click on the 'media' link and then 'add media'.
Finally, you choose which file/photo you would like to upload from your computer and it will take a few seconds to upload.
You then have the option to add further information about the photo and to decide if you want it to be the main profile photo for that person in your tree.