I ran into a problem recently when doing some development work for the iPhone. I have created some images with transparent backgrounds for use on some buttons in an iPhone application. The images showed up perfectly when I was testing my app in the iPhone simulator. But when I transferred my app to an actual iPhone, the images were messed up. Instead of a transparent background, the background was solid black. Needless to say, this did not look so good.

The fix I found was to open the images in the Preview app and then save them again without making any changes. When I did this, the transparent background was then actually transparent when the app and the images where transferred to my iPhone.

Not sure if it makes a difference or not, but the original images were created using GraphicConverter. So if you're images are not showing up correctly on the iPhone, try opening them in Preview and then save them again.