You can't just do this. Images have rights holders.
OP, what you can do is look for Creative Commons pictures. These are usually free to use, as long as you credit the image creator in your description. A lot of people do this. My favourite one is Flickr Creative Commons. Make sure you search through free for commercial use pictures if you want to monetize.
My general thumbnail process is:
* Find background image (usually from the game I'm playing)
* Find a "key" image. This can be a game's character, your head, or something else
* Create the title
Sometimes there's more, sometimes (rarely) there's less. This is my minimum checklist of things. You can find heaps of tips in the forums for how to go about treating the image to make it look good.