Changing your wallpaper on a iMac or Macbook Laptop is pretty simple. Here I will explain the process on how to change you wallpaper with an iOS Mac computer.
STEP 1
Select the apple icon at the top left of your mac desktop screen. Then select "System Preferences..."

STEP 2
Select the "Desktop and Screensaver" icon.

STEP 3
In the next menu screen, select the image you wish to change to your new wallpaper and its desired preferences. If you do not see the wallpaper you wish to add - select the "+" at the bottom right to choose the folder that the wallpaper you want to use is in.

Once the image is selected it should automatically change on your current desktop.