the ones i have that seem to work are the following resolutions:
some are ones i've taken with the panoramic feature of the camera app, but others were just normal pictures that i'd previously cropped to very widescreen ratio a few weeks ago. When i installed iOS7 it must have recognised them as a panoramic shaped picture
You can take any hi res image from the web and make it a panorama by simply cropping it in the photos app. Sometimes it takes a couple tries to get it right but as soon as you do it'll pop into the panoramic folder immediately.