The software for driving the slide printing device is capable of printing graphics files directly, without every putting them in PowerPoint or any other application. Just be sure to send your files in a useable format, as discussed below. Raster graphics are collections of individual pixels, such as scanned images or images from paint programs. I recommend TIF format on the PC platform, as it allows you to retain all your original resolution and color depth. Vector graphics are composed of mathematically-defined lines and curves created in drawing programs like CorelDraw, Freehand, or Illustrator.
Fonts are also vector graphics.
One nice thing about vectors is that they will print or display at whatever resolution the output or display device is capable of, so on a slide or high-resolution printer you get very smooth lines. Plain old postscript, as opposed to the encapsulated variety, may not work.