As a caveat to the using decal paper, some good USAF graphics for use with a photo editor to set up your decals can be found here on the USAF website art section:

http://www.af.mil/art/mediagallery.asp?galleryID=5147

As far as photo editors to set up decals (or any image generation really), Photoshop if you can afford is best as it can set up your images using CYMK colors for printing.  But, most of us mere mortals cannot afford that, so I use the GNU Image Manipulation Program (GIMP) which can be downloaded for free from http://www.gimp.org/  You can go into the preferences and set up your image document to match whatever size of sheet you need to print.  GIMP is limited to RGB colors for now, however you shouldn't experience too much of a difference between CYMK/RGB with decals...  If you don't want to risk it check out Scribus http://www.scribus.net/canvas/Scribus to assist with conversion.  Other free photo editors worth looking into for the decals would be Paint.Net, or if you are a Linux guy (specifically a KDE user), Krita.