Hi. What kind of decals are we talking about?
Large scale decals that don't have to fit precisely on tiles or bricks can be arranged with Microsoft Word or OpenOffice Write to place as many as needed on a single sheet.
Smaller or detailed decals (like custom maps, armor, shields or signs) are best made with a typesetting or page layout program (Adobe InDesign, or the open source program Scribus). These allow you to specify the exact size of each decal (up to micrometers - here's the formula to calculate actual brick sizes: dimensionsinfo.com/dimensions-of-a-standard-lego-brick
) and to draw custom outlines/shapes which can be filled with solid colors or images.
If you know what you are doing you can do all these things with GIMP (gimp.org
) as well.
“Accept His Noodly Magnificence into your heart, into your soul, and ye shall forever be free. R'Amen.” – Ragu