Help! How do I.....

[live4christp1]

....fill in the background color?

Okay, I have several beautiful designs from you wonderful designers here on the DIS. There are a couple that I want to fill in the background color to the color of shirt I'm putting them on. I've tried in paint but it's very tedious to color in around all the characters.

Is there an easier way to do this?

Thanks,
Cristy