The photo sync is one-way: Computer to phone. If you sync photos to your phone, then delete them on your computer, there is no supported way to extract these photos from your phone & then add them back to your computer. Further, photos synced to your phone are reduced in resolution for display on your phone. There is no way to recover the photos full resolution, should you delete the photos on your computer.
Bottom line: Don't use your phone as a storage device.