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Press on the image and hold, a box will appear with options on what you can do, select Save Image and the picture will go to your camera roll.
 
That worked with my iphone 6, but does not work with the 7
Thats strange, as I've just been doing exactly this on my new 7. I have a few important photos on my iPhone5 with I didn't want to risk loosing through a back up restore to my 7, so I emailed them to myself from my 5 and opened the emails on my 7, held my finger on the image until a box popped up with options on what to do with the photo, I just tapped on Save Image as below and they go straight to my Photo roll...

ff203e45-6b3e-4930-bfdc-267d9bafdcd4_zpsvpo1ivas.png
 
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