I think you do need to explain this to the developer. I doubt you'll find some magic solution here, and talking to the developers is basically what being PM is

I think you do need to explain this to the developer. I doubt you'll find some magic solution here, and talking to the developers is basically what being PM is all about. Communicating with people is as central to a PM as code is to a developer. If after explaining he keeps repeating the same mistake, perhaps the problem's with him. If you find this happens with more than one developer, perhaps the problem is with you. You might decide to change the process - pass the mockup to a designer first, and then the designer's output to the developer.