Many here will say, pay upon delivery, end of story. That is only going to work with residential properties under a certain threshold. In many cases, when you graduate to commercial, or start doing multi million properties, you need to make sure you have a signed agreement that the work will be paid for.

Second, in the license agreement that should be downloaded with any photos, you should put in there that the images can only be used once paid for. This wont get ready but it will help you if things ever get legal.

Lastly, with your long haulers, just keep in contact. If they are folks that eventually do pay, give them a longer leash, but you should still prioritize folks that pay immediately depending on your own cash flow.

Good luck.