To upload pics: open your UserPC. In the left sidebar, under "Networking," click the button titled "Pictures & Albums." Click the "Add Album" button, title your album according to what pictures you'll be putting in it, and start uploading!
To add a horse to your profile (or barn): In the navigation bar, click the link titled "Horses." Visit "Your Barn." Click the "Add a Horse" button. Everything else is pretty straight forward from here on.
If I'm as clear as mud, let me know and I'll try to explain better. :)