I am looking for a Joomla template - thing is how do I know that the background images used in the template are licensed or has the designer just pulled/nicked the image from elsewhere - thus making me liable for any comeback?
That's a good question about any theme/skin, whether it's Wordpress, Joomla, Drupal, etc.
To a large degree you are relying on the integrity of the developer of that template. If this is a free download, then try to deal with reputable designers who are visible in their software community and have been offering this or other designs for a period of years.
If you are buying a custom design, then be sure that the developer certifies that all images are original or properly licensed.
One sure-fire way to be certain is to actually change out background images with ones that you acquire yourself. Typically, much of the effort in template design goes into factors other than the background and you might be able to find a similar image for low cost at sites like istockphoto.