Kindle usage: Breakdown by device type?

Does anyone know of any recent survey or research data that describes the breakdown of Kindles by device type? That is, how many people use Kindle Touch vs. Fire vs. older models, etc.

I am working on resizing interior images and want to know how backwards compatible I should strive to be ... old Kindles have a 520px width, while the Fire is much larger. I'd prefer to go large, but if there are too many older devices floating around, it makes sense to stick with the smaller image dimensions.

Thanks

Ian