Images could be chosen from the following:

1. Designer Stocks
2. Supplied by customer
3. Produced in house
4. Public Internet Stocks
5. Public Domain

Image price is equal to image plus search time.  For example if an image cost $50 dollars
and it took two hours to find it, modify it and place it.  The total amount will be $90 dollars.

A good image is the one that represents your product and give the intended impression to the possible customers.  It also has to be crispy, and well composed, and carry the right colors.  In most cases, colors can be modified and the image can be lightly enhanced.