I have an iOS6 iPhone application, with a Tabbar and tabbar buttons. These buttons need icons and that's where the trouble starts. Added one icon to the project, double clicked on a button in the designer and associated the (questionmark) icon via the image combobox to the button. When I run the app on my iPhone, I do indeed see that the button now does indeed display a gradient-gray square, which turns into a gradient light-blue square, but the questionmark icon is not shown. The icon is a .png and the size is width 40, height 51. I changed nothing in the code.
I solved my problem to the extent that I can work with it: you need so called 'transparent icons'.