When sending images to your customers, it’s very important to make ordering as easy as possible. The biggest problem is to remember all filenames they want to order. A combination of IMG or DSC with a random number isn’t somewhat easy to remember for you and your customer.
It’s a good idea to sequentially number images before sending them to your customers. It’s so much easier for them to say they want images 4, 8, 15 and 27 than look into filenames and say they want files DSC02154.jpg, DSC03153.jpg and DSC15345.jpg.