Using a neat little website (placeit.net), you can place your book covers (or other product images or website images) into interesting settings: presentation settings, mobile phones, computers, t-shirts, even fashionable videos.
Check out these two images I created with PlaceIt.net.
As you can see above, the image is free to use with no advertising.
That is not true for the free videos, which include a PlaceIt brand over the video. You can pay $29 per video to remove the PlaceIt brand.