Everyone here seems to favour vector graphics. I assume that this is for side art and even CP art since a photoshop file at 300dpi (professional print resolution) is a pretty heavy file. if your cp is 28" x 14" thats 8400 x 4200 pixels. I've done D size renderings for work (architecture) at 150 dpi and the file was around 17 megs. The more layers you add in, the heavier the PS file.
So, that being said, vecotor graphics are not resolution dependant. That means that you can zoom in a million times and a sharp line will always be a sharp line instead of a big black pixel with a jaggedy edge that fills most of your monitor. Conversly you can have a normal size file and blow it up to side art size and it will not loose print resolution.
Ok. Now, Adobe Illustrator is the cream of the vector graphics programs out there.
If you're doing your marquee you can probably use just about any image software. As long as you understand DPI you'll be able to print something which looks nice. Shoot for 150 dpi. You won't be able to tell the difference between 150 and 300 in most cases. I wouldn't go any lower. Be careful of any images you download from the 'net. Internet resolution is 72 dpi to keep file sizes down, sow images download faster.