You can put in both an avatar picture and a profile picture (which appears on your one page site profile, but not your Member Page profile, which is a separate set of pages).
You add in these pics by going to your User CP (top left of orange bar at top of the page) and follow the directions. If avatar/profile pics are within the right size range they are automatically sized to fit the avatar/profile slot.
Please note that you CANNOT post a picture in your signature -- mainly because there are plenty of places to put additional pictures of your dog(s). If you try to post a pic, all that will come up is a clickable link. Better to use the sig space for a proper sig!
Go to your Member Page, though, and you can upload several photo blocks and place pictures there, though keep in mind the files need to be small -- under 200kb. The smaller you keep them, the more pics you can put up as each person has a designated amount of storage space for their Member Pages.