The helicopter game should be dodging missiles not clouds 🙂
This is also in DropBox – in sketches\images
This is a PNG image – it has a transparent background.
The best way is to find an image that already is PNG and has a transparent background.
If you find a JPEG – you can down an application call Paint.NET – its free. http://www.getpaint.net/
Load up your jpeg and then use the magic want to select the background you want to remove. Then just delete it and save as a PNG – Magic!