Are you using the stock email app?
I haven't really used the stock email app since I have always used k-9 on my phones
k-9 I think is way better than the stock app and easier to use once setup. For pics in k-9 once the pic is downloaded to the email there are 2 buttons next to the photo open or save.
Edit: wait i think i remember answering this in another thread. After you download and open the photo to look at it when you then go to your gallery is it in there? I believe once you have downloaded it the phone automatically saves it.