There are a number of tools for creating decals for ldr files.
http://www.melkert.net/test/stickerGen.html
https://github.com/nilsschmidt1337/ldparteditor/wiki
http://www.scottwardlaw.com/sticker_generator/
But I struggled to find one that would allow for seamless transfer of stickers between LDCAD, Studio, and Blender. Consequenly I ended up using svg2ldraw
http://c-mt.dk/software/svg2ldraw/
For me svg2ldraw works well but it requires a fairly detailed understanding of vector graphics. I was using an old copy of Illustrator CS2(!) to manipulate the original vector artwork (but Inkscape may also work). svg2ldraw allows you to remap colours into Lego colours (including transparency) which allows for separation of colour elements from the background when placing the sticker in Ldraw. The image below shows the original design in Illustrator, the sticker in LDraw and the final render in Studio.