Help creating decal rendering

[SD455]

I am trying to, for the first time ever, create a stripe kit drawing for my firebird. I am wanting to do something with a photo that I can then send to my decal guy so we can work on creating the actual decal. For obvious reasons I want to play with a few different themes etc and come up with something I like before taking it to someone else for completion.

Can anyone tell me what free, or reasonable software I can use on my Mac to do this? Again I am a complete beginner.

Thanks

B