It all depends on the size of your stage and what you plan
on doing with the graphics. If you will not be zooming in on your
image save it at 300dpi. If you plan on making the image larger
than its original size I would say make it 600dpi, then you can at
least scale it up twice as large without it getting pixelated.
Scaling down is not a problem, it's scaling up that can get you in
trouble. If your image has transparency then save it as a PNG file.
That way you'll be able to use all of your transparent areas
effectively. I usually just save my files as PNG anyway then if I
decide to go in an mess with them later on I won't have to convert
to a different format.