This is a very simple thing to do but it seems I run into many people who dont understand it yet so I thought I would try to make it clearer.
Start by opening a photo in your favorite photo app. The gallery app on most devices works well.
In my case the g+ icon is already shown because I use g+ often. If yours is not showm, click the share button.
This will open a menu with all the apps you can share too. The list will have many things like g+, Gmail, other email programs, etc.