Yes, You can create better looking ones with gimp (free). An good graphics software is capable of doing it. You just have to learn how to do it...
Spent a few minutes searching for such software. could not find one that is free. It is easy to create such images, if you know how to draw rectangles, place images in it and know how to skew them.
try to use the online photoshop version if you don't like gimp or hire a desinger I would go for photoshop definetly but no online version bc you can do things easier if you are not forced to use the ways a script does
The most important part id the flat cover (parts) design. It's not that easy, as the cover has to look good in smaller sizes, and it requires different approach than when designing a cover for print. Once you have your flat parts, they can be made into a 3D e-cover using any graphic program. You can also use http://www.3d-pack.com/.
there a lot of tutorial which you can find it if you start googling around..or just purchase any action script and design your own ecover..