If you are having the bike custom painted, not doing it yourself, take a picture of what you want to the shop and ask for ideas/mockups along those lines. A modern custom shop ought to be able to do this, I would think.
I use Gimp (free) for making logos, etc. But I am a novice and there is a learning curve to using the program. The price is right if you have time to play around with it.
This would be fairly easy for one in the know to do.
Funny that you say that. I did do a mock up using Gimp. I was not liking my paint scheme. I think I will redo my idea. I think that I will have to leave about a 1/4 of the tank black and follow that line down to the side covers on a curved line. The saddlebags will be half and half, but have the paint line in an arc instead of straight.
I was thinking of doing this myself, since a paint job like that would probably be $2000 - $3000.