what about nine-patch images.
Can they be used instead of the ?
/res/drawable-hdpi/icon.png ….. 72×72
/res/drawable-mdpi/icon.png …. 48×48
/res/drawable-ldpi/icon.png …… 36×36
What are the benefits/drawbacks?
Technically you don't ever have to provide an image for all three resolutions- if one is missing, the system will replace it with one of the others. Though it's usually better to have all three for maintaining appearance across all screen densities.
9-patch can be used instead, but should be only under specific circumstances. If you have an image for a button that expands but preserves the corners, keep in mind these corners are going to look much smaller on a high resolution screen if you don't provide an hdpi version. But the size of the image itself will still expand according to the specified stretch areas.
So in a nutshell: 9-patch does allow an image to expand as needed independent of resolution, but the non-stretchable parts of the image will not be magically converted to higher/lower resolutions.