Welcome to Android Central! Remember that the Gallery isn't a location--it displays all images in any directory on the phone or SD card, as long as there isn't a file called ".nomedia" in that directory.
Use My Files or any other file manager app to move picture files from the /DCIM/Camera directory (if they were taken by the phone's camera) to the microSD card. Or connect the phone to your computer via USB, and use Windows Explorer to drag and drop the files.