You may add one banner on the top of the page and two pictures to the site of the home page greeting. Further, you are able to add pictures to the scrolling banner on the bottom of the page (sponsors).

Top banner:
General Setup > Non-Profit Information
Maximum logo size (width x height): 600 x 100. Recommended: 400 x 100

Side pictures:
Online Auction Setup > Home Page Greeting

Scrolling Banner:
Scrolling Banner > Add Banner