[Brian]

Well, OmniGraffle can handle pretty much any QuickTime compatible graphic format as a shape, but the thing to remember is that each graphic is a shape.
(The designer will want to save each shape to a document, in other words, rather than one document with all the shapes.)

Just off the top of my head, I'd say to save each of the shapes as a PDF, so you can resize them/scale them nicely. Note, however, that PDF graphics can be quite large, so if you plan to use these stencils on an iPad, that may not be the best choice.

Does that help at all?