If you have 3D touch (which does come with the 2nd generation of SE), then you touch the image once, don't let go (it will fit the screen so you can see the entire image), press down harder and the image will open in a page for itself. Only after that will you see the download icon. Tap the icon, then select 'Save Image'. You can then find the downloaded image by opening your iPhone's Gallery App, tap albums, and navigate to Downloads. (also, if you're looking for an app called the Camera Roll, you won't find it. Photos is the new app that contains your camera roll as well as other photos that you've downloaded or screenshotted)