Shapes and polygons can contain images in addition to their regular fill. An image can be strecthed or tiled to fill the graphic, or it can be scaled and offset so that only a portion of the image is visible.
OmniGraffle accepts at least the following image file types: PDF, TIFF, JPEG, PNG, GIF, and PICT. In general, any image format that can be imported into TextEdit or Mail or Preview should work in OmniGraffle as well.
You can drag in graphic files from the Finder, or drag images from any application they appear in (if that application allows it). Or you can select and Cut or Copy a graphic in another application and then Paste it in OmniGraffle.
When an image is dragged over a graphic that can accept it, the graphic will highlight. If the image is dropped onto the document (not on a valid graphic), a new graphic will be created that is the same size as the image, using the image as fill.