- Create decal in any art program, say 300 x 300 pixels size, save with a transparent background as .png
.  Note saved location!
- Load a Character in CC with something like Shirt A, ,goto Appearance Tab, highlight Shirt, Activate Appearance Editor/Decal1.
- Goto Diffuse then goto Custom box, click Custom and a Window will open, navigate to your saved Decal, select it and click OK.
- Decal will appear on the shirt then use positioning/size tools to relocate Decal to desired position and size etc.

I don't believe there's such a thing as a transparent JPG...  Just sayin'

Thanks for the tutorial, however I don't have a 'Custom Box' under my decal. I have tried all the default characters and don't see it listed under any of the 'Decals' 

U need to purchase the Essential pack. I had the same problem, but appeared after i downloaded it.  

Thanks Wizaerd, I learn something new every day!