Yes, you can use graphics from any source. You are not limited to using only the iScrapKit artwork that comes with the software or that is available from iScrapbook.com. iScrapbook works with any graphic file so long as it's in a popular file format like jpeg, tif, png, pdf, gif, etc. This means you can use images you already have on your computer or images you legally downloaded from the Internet.
There are three easy ways use graphics stored on your computer:
Drag & Drop. You can simply drag the graphic file from the Finder to the Design Canvas (your scrapbook page).
Favorites Browser. You can add the graphic to the Favorites Browser where you can easily and conveniently use it whenever you want. You can organize your graphics into collections in the Favorites Browser to make it easier to find and use graphics. For more information on the Favorites Browser, see the Favorites Browser chapter of the user's guide (page 79 as of this writing).
Insert Feature. You can choose Insert > Choose Image and then choose the image you want to put on the Design Canvas.