First of all, Photo Stream has to be turned on in your iCloud account before you do anything.
In the built in app, tap and hold the picture until you see this:
Here, choose "Save to Camera Roll". The picture should automatically be added to your Photo Stream.
I didn't find a way to save pictures from the Gmail app directly. There's another one, Mailbox, where you can do it:
Also, tap and hold the picture and see this: