You can download a theme from the Samsung app store and that will change the image on your home screen. To access themes hold your finger in any open area in the home screen and the theme option should come up. In addition to that, you can select a photo from your Gallery and tap it and then tap the 3 dots on the lower right and a menu should come up with the option to set it as wallpaper.
I used the zedg app to change the background on my phone besides the galaxy themes the images in zedg will only change the wallpaper on the home screen and the galaxy themes will change the home screen wallpaper and app icons
If you want to use one of your own photos that are saved, or use bing images, press and hold on your home screen and it'll zoom out and let you adjust wallpaper (home screen photo?) as well as widgets.
