Open your Camera app and tap the cog symbol to open the Settings Menu. From there simply select 'Storage Location > Memory Card' and any pictures captured form that point onward will be automatically stored on your SD card.
Any pictures you take from now on, with the storage location changed to 'Memory Card', will be stored on the SD card but will still be visible in your Gallery alongside your other pictures. To check they are being stored successfully please take a few sample images, remove the SD card and place it in another compatible device (or card reader on your PC). Are you able to see the sample images?