Commands comming to you from the Drawing toolbar:
To make a perfect circle chose the "Oval" icon, start the build while
holding down the shift key. While it is still selected use the "Fill
Color", "Fill Effects", "Picture" tab, "Select Picture" button.
Commands outside of Powerpoint:
Probably the "My Pictures" folder will open and you can make your fill
using one of those. If the picture is in a different location you may
have to use Windows + f and do a search. Or it may be on the internet
just waiting to be portrayed in your presentation.
Hope this helps to scope out an image.