You've already mentioned the correct article for this problem. There is written:
res/drawable-mdpi/my_icon.png // bitmap for medium density
res/drawable-hdpi/my_icon.png // bitmap for high density
res/drawable-xhdpi/my_icon.png // bitmap for extra high density
res/drawable-xxhdpi/my_icon.png
Additional you could add normal-long like you said (but I don't think this is necessary):
res/drawable-normal-long-mdpi/my_icon.png // bitmap for medium density
res/drawable-normal-long-hdpi/my_icon.png // bitmap for high density
res/drawable-normal-long-xhdpi/my_icon.png // bitmap for extra high density
res/drawable-normal-long-xxhdpi/my_icon.png
Also you need to consider if you maybe create folders for different orientations (land, port)
Edit: You should stick with Xoxols answer: android:scaleType should do the trick to get rid of gaps on top and bottom of the screen.