This site I have used for quite some time and Is such an epic site.
you do have to sign up with the site to get full size Images,
But here you can search for stuff and get pictures without backgrounds, This way It Is easier to copy and paste Into your signatures to make some cool backgrounds.
Planet Renders v4
Just sign up and search for what you are searching for.
here Is just a quick tutorial

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When In photoshop open up the eraser tool (for CS4 users hold click on eraser) and change It to the " Magic Eraser " and click the background.
It will remove the background from the Image and then you can copy and paste the Image Onto a signature that you created.