You can't really do this - I tried once when I accidentally deleted quite a bit of my photo library. All photos in "albums" on the iPhone are encrypted into one database file that isn't able to be read or decrypted by a computer; just the iPhone. The photos actually taken with the phone are the only ones stored as image files.
However, there are iPhone data recovery apps such as
this one that can pull things like photos from the iPhone to your computer. The only downside here is that there quality will be decreased.