Okay, you can import image as custom background in Videoscribe, but resize the image so it can fit the camera is a pain in the ass. Because once you make it larger, the camera will zoom-out too. So my suggestion, put a button where you can make your image 'fit to camera', it will instantly resize the image to the camera size, and then people can just lock it, make it as custom background. I think this is very easy to implemented and can help people to have 'custom background' in just couple short click.

Dean Farrand