1. Get the source
2. Place your picture
Choose layer named “place your pic here”. The upper purple layer represents place reserved for clock and another information on the lock screen.
Open Finder and find a picture you want to use as a wallpaper. Drag this picture directly into Photoshop. Another quick way is to use menu item File > Place…
Center your picture
The picture will appear in center of our file. Now a trick: to zoom out hold Cmd + Space and move your mouse (or finger on touchpad) to top.
We need to match the hight of the picture with the height of the file.Also hold Shift + Alt (this combination prevents distortion and lets you transform the object from center and not from corner) and drag one of the corners of the inserted picture out of the file`s rand.To get a transformation box anytime, not only after placing the picture – press the great hotkey combination Ctrl + T
Delete guide layer and save
Delete the layer “delete this layer” which is above the layer with your picture, marked with purple.
Now go to menu File > Save for web… (Or press Shift + Alt + Shift + S)
Choose JPEG (if it is not chosen already) and press button “save”
Now type the wished name of your file and press “save” again
What to do if the picture is too small to fit the whole screen? This will work fast and easy with images on solid background. Choose the Eyedropper Tool and pick color from the background of your fresh placed picture.
Then choose layer “Background” and press an icon “Create a new layer” in the bottom part of your layer panel. Another way to make a new layer above the chosen one is to use hotkeys Shift + Cmd + N, then OK