As @Gagea25 indicates,
The short answer is NO IMPACT! You can delete anything in the screenshots folder at any time, including when the game is still running, with no effect on your in-game photos. The images for photos are stored in the save itself. It's the same as the celebrity signatures and the DU and HSY graduation pictures once the photo is in your sim's inventory. If you take a good look at the screenshots in the folder, you'll notice that they do not exactly match the corresponding photo in the game, they show a wider view than the in-game photo.
One word of caution though. If you have a crash before you save and exit the game, recently taken photos, celebrity signatures and graduation pictures will turn solid black, and no recovery is available. That includes force quitting the game, even if you saved between the time you took the photos and when the crash/force quit occurs.
Imo, what would be really nice is if you could import pictures in the screenshots folder to be used later on as framed photos or paintings. Then we would be able to recover our photos if they do go black. Sadly, that's not the case.
Hope this helps.
