The copyright holder is the person who uploaded the image, and that person gets the credit. On Shutterstock, when you click on an image to get the details, you'll see Image ID, Copyright, and Release Information (where applicable) listed right above the Download button. Credit should be given to the user listed under Copyright. For example
this image should be credited (C) IgorGolovniov @ Shutterstock. Just be sure to keep your payment receipts so that, if an image is ever called into question, you can provide proof of payment for the image, and be sure to continue to follow the photographer's rules- for example, even if you paid for that image posted above, you would not be allowed to profit from anything you make with it because the use is non-commercial only.
I'm not sure which other sites you used, but they should list similar information.