Final answer:
To save photos to your camera roll, transfer them from the camera to a computer and then to your mobile device, either through a direct connection, cloud service or email.
Step-by-step explanation:
When you want to share your photos or save them from files to your camera roll, you can download the photos from your camera to a computer. Typically, when you connect your digital camera to the computer, the image files will be accessible in a folder named DCIM. Inside this folder, you can copy the JPG images and paste them into another location on your computer.
If you wish to transfer these photos to your camera roll on a phone or tablet, you could do so by connecting the device to the computer and copying the files directly to the appropriate folder on the device. Alternatively, you can use cloud services or email to transfer the photos from your computer to your mobile device, where you can save them to your camera roll.