Not necessarily one .png - just one download with all the wall tiles .png or sprite sheet in it.
And a separate download file for the props etc.
So you'd have a characters.zip / a props.zip and a tiles.zip files - and you can make 3 store listings, or put it all in one and people can choose to download all 3 or choose any 1. If one download is more popular than another it will also give you an idea of what people like the most about your stuff.