Hey so just clarify something, photodune is strait stock photography. Alpha channels and layers (Transparency in images) are graphics attributes which is why they can’t be found here, but rather on graphicriver. Whether or not there’s a way to search for images with an alpha channel on that marketplace, I’m not sure.
If you’ve found an image on this marketplace with a strait white background you’d like to use. Making it transparent with photoshop or gimp is really easy, and I’d be happy to walk you through the steps If you need help with doing that.