Well, it really depends on quality.
If your image is just a square border, it can be a 72 dpi low res image put into the drawable folder and it would be enough.
If your image has rounded corners or other fancy elements that have to be scaled properly, you could make a 480 dpi version and put it in the drawable-xxhdpi folder. This will scale down (don't even think of scaling up, because of stretching/pixellating) good enough in most cases.
If you want the best quality in scaling, then make a version for each dpi drawable folder.
If you go to http://developer.android.com/guide/topics/resources/drawable-resource.html, you can see it there (citation):
file location:
res/drawable/filename.9.png
The filename is used as the resource ID.