What deliverables do I need to request from a visual designer for my Android app?

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I am designing an Android app game for children. I have created wireframes for each screen.

I am used to requesting graphics with specific pixel sizes. But I believe that Android uses 'density-specific graphics', not pixels. A lot of the information I have found via other answers -- e.g. http://blog.stylingandroid.com/archives/327# -- is useful but too technical for me.

a) In what exact formats do I need to request the graphics, to maximise cross-device and cross-form compatibility?

b) Should I request one graphic (multiple formats/states) for each shape? And also one graphic for each screen background; one for each button; etc?