|Adding graffiti via Projected Light emphasizes boobies (erm ... s'what the graffiti says)|
Last point is about the mechanics of the source prim influence the size and shape of the final projection. Check out this info graphic:
Aside from the position of the prim along it's Z-Axis, rotation is key, too. If you don't rotate the prim at all, it will project on the floor. Setting the Y-Axis to 90 degrees projects the image to the west (thus 270 degrees would be eastward, and so on) and another 90 degrees on the Z-Axis makes text read left-to-right.
So those are my helpful tips. Now get out there and build! Feel free to post links to your pix in the Comments of this post. I'd love to see them.