Windows 8 is even easier.
As with many things in Windows 8 it’s different than Windows 7, but it is, indeed, easier.
It starts with Registry Editing
Don’t let it scare you but all this takes is a single, simple addition or change to the Windows registry.
Fire up regedit, most easily by clicking on Start, then Run, or typing the Windows key + R, and typing regedit in the resulting dialog:
You may need to confirm a UAC prompt.
In registry editor, expand by clicking the small triangle to the left of each:
Look for a key underneath Windows called Personalization. If it exists, click on it.
If Personalization doesn’t exist, right click on Windows, and in the pop-up menu that results click on New, and then Key:
The new key will be created with a default name, but in edit mode – replace the name with Personalization.
Now, right click on Personalization and then click on New and then String Value. Give the new item the name LockScreenImage.
If an “Edit String” box doesn’t immediately appear, or if Personalization was already present, then right-click on Personalization and click on Modify. The edit box will appear:
In the “Value data:” field, type the full path to the image you want to use as the lock screen background. Ideally the image would be the same width by height as your screen’s resolution (1600 x 900 in my example).
That’s it! Log out and you’ll see your new lock screen.
A quick video walk through of the process, step by step: