If you want to do custom graphics, have them transferred to a transparency (any Kinkos can do this for you). Have them print it in reverse
, so that when you place the graphic against the body, it's printed on the side that will touch the guitar. Use an adhesive that's specifically designed for mounting photos (3M makes a good one, it runs about $12 a can at most office supply stores).
Trim it so that the picture has about 1/4" of the transparency surrounding the graphic. Then, using some 600+ grit sandpaper or some 000/0000 steel wadding, rough up the area you are going to apply the graphic to. The 3M spray adhesive will not stick to a completely finished surface.
Apply the adhesive to your graphic, stick it to the guitar over the area you roughed up. Use a credit card or something to smooth out any bubbles. Be careful not to scratch the guitar in doing so. If you have any adhesive overspray or mess to clean up, a little Zippo fluid will clean it up.
Then, get a good clear coat
(Lowes or Home Depot sell a couple decent sprayable clear coats) and start clear coating the decal. Deft is a good clear coat. Lightly build up coats of it - do not use any heavy coating.
That should do the trick.