OK. Fine. So I don't know what you're thinking. But it doesn't matter. This is supposed to be about what I am thinking anyway.
The iPad screen is 1024x768 and you might imagine that any 1024x768 image will do the job. The problem is that the iPad screen is also 768x1024. Confused? Basically, this is the difference between Landscape and Portrait orientations. So we need to create an image that is as large as each orientation's longest side. A 1024x1024 picture is what we need.
This piece of information might be all you care about at this point. But if you are like me, you will soon realize that there is another factor to consider. Apple overlays three bars across the backgrounds (at least on the lock screen): the status bar (where the carrier, signal strength, etc. is displayed), the time and date area and the unlock bar. We probably want the most interesting parts of our backgrounds to miss these areas and remain unobscured.
And one more twist. The size of these bars are different (both height and width) depending on the orientation. Lovely.
Confused yet? Well a picture is worth a thousand words (lucky for you).
Click the image to see it full sized. You might even want to save it somewhere to use as a template.
If you decide to use this as a template, you can overlay a picture on top of it and reduce the opacity enough to see the boxes. By scaling and resizing your image to completely fill the 1024x1024 square while keeping the "important" bits inside the "sweet spot" area, you should end up with a background that works reasonably well in either orientation.
And the following shows the end result in both Portrait and Landscape.
Of course, you can always just guestimate.