Here are a few key takeaways from my past experience on designing for mobile devices..
1) Responsive design
2) Create multiple graphics based on resolution for different devices
3) PNG format works better (has greater color depth)
4) Liquid design to tackle G-sensor for portrait or landscape setting
5) Keep the file size lighter
6) Use relative over absolute sizes
There may have other points as well however, I suggest, keeping tab on these points (while designing graphics for mobile devices) will help one immensely.
Addressing the small details means delivering optimized mobile apps and websites – both technically sound and are providing super UX.