I get payed through Paypal. It's safe, comfortable for almost everyone and takes no charge (as far as I know). They give you some protection, but to keep safe the best is to handle the customers yourself. The best way I found was: show progress steps of the image, so they don't feel scammed, and don't deliver the finished piece before they pay. Ever. Show them the finished piece in loooow quality, or watermarked, so they're sure you've done it, but never give it away before the payment.
The best would be to have half of the payment upfront, but it might be discouraging, so just be straightforward with the customers and find an agreement with them. You'll discover most of the people have the same requests, and you'll develop a routine that works best for you