Desktop Wallpaper keeps reverting to an old image every time I restart

Try Settings/Personalization/Background, there you can set a Pictire as background, or browse for one.

If you want to kill off the "Black Camaro" try looking here for any image of it.

Where to Find the Windows Wallpaper Location on Your PC
http://www.tekrevue.com/tip/windows-wallpaper-location/

Desktop Background - Change in Windows 10
http://www.tenforums.com/tutorials/4966-desktop-background-change-windows-10-a.html

See also, HKEY_CURRENT_USER\Control Panel\Desktop, look at the value @ Wallpaper

Where's the wallpaper registry key in Windows 8?
http://stackoverflow.com/questions/11278707/wheres-the-wallpaper-registry-key-in-windows-8

Don't be put off by things that mention Windows 8, or Windows 7, as they are just as likely to apply to Windows 10 too.  In fact you can find Windows 95 things that still apply if you look hard enough.

If you run Disk Cleanup there are options there for cleaning up Thumbnails and other Temp files which might help you.

Here is how to fix your wallpaper changing back or changing. 

1. Right click on the desktop and choose Personalize.

2. Click on Themes and then Theme Settings.

3. Look under my themes and see if the selected theme is the "Synced Theme"

4. If it is that is your problem. If it isn't change the desktop picture and save a new theme name. 

If you want to change the auto backup of your theme, then go to Settings, Accounts, Sync your settings, and then turn the toggle for theme to off. 

This should fix it. 

Hello,

In some laptops or netbook like this one https://www.asus.com/Notebooks/U36SG/, you need to change the power saving mode.

Just hit the Power4Gear Key on the top left side of the keyboard to High Performance.

You can't meaningfully delete the Synced Theme so long as theme synching is on.  You would want to go to

Settings :: Accounts :: Sync your settings

and turn off "Themes".  Then make your own theme changes, and those should stick.