Well, I think you need to give more information about what you are exactly building.
1. You talk about 3D first (altough to my knowledge, shutterstock deals with 2D images?), then you start talking about PSD and image quality. Are developing a 3D or a 2D game?
2. Does your developer use an existing engine? Selfbuilt? Modding an existing game?
3. If we are talking about 2D Games, first thing you need to be aware off is that some games, especially in the flash scene, use vector graphics to achieve this nice, crisp look, versus bitmap/pixel graphics. If you don't know the difference: bitmap graphics formats like PNG or JPEG save the image formats as a map of pixel color/alpha values... vector graphics formats do so by saving coordinates and mathematical expressions that make up basic shapes. https://en.wikipedia.org/wiki/Vector_graphics
4. There are so many things that can go wrong with pixel formats, its not even funny. The biggest keyword to search for would be scaling images. Even a high resolution image that gets scaled ingame can look horrible. 2D devs might be more helpful than me here, but AFAIK that is the reason why a) you scale the final sprite to conform more or less to your target resolution, 2) you are better off NOT scaling yur sprites ingame because there is no way to prevent artifacts without swapping the sprite for one with a different resolution, and c) if you target multiple resolutions, you should test each and maybe even have sprites in different resolutions.
5. Last thing that comes to mind: you send your developer a PSD images... while SOME engines allow you to import a PSD images, the running game will need an actual bitmap image format like PNG or JPEG, TGA maybe... PSD is a big, layered image format proprietary to Photoshop. Many other programs support it thanks to PS popularity, but it is certianly not useful as ingame sprite format.
If you just send PSDs, maybe ask your dev what he does with these. At some point they need to be converted, there could be all kind of stuff going wrong during this conversion process.
And nothing against you, but paying someone after just 2 years of research/expierience to develop a game for you sounds like quite the adventure, especially when you seem so inexpierienced in both technical and art topics. Maybe try to improve your knowledge on these topics instead of relying on offshore people to know better, even if you are only taking on a producer role.
Again, meant as good advice, not trying to judge you or anything.