First off - if you're saving it as a 256 color bitmap, it's going to have .bmp as its extension, not .png. You can import it like that. Secondly, are you actually importing it into the Tileset folder? While in the resource manager, on the left side, you want to click the "Chipset" folder BEFORE you try importing. (Why is it called Chipset and not Tileset when it comes to the folder? Lazy programmers).

As to prerendered backgrounds, err.. that's the whole subject at hand, right? Unless we've had a major communication error, that's the whole point of using the tilesets and such..