Apple asks that your UITabBar icons be pure alpha-channel images with added anti-alias. That means that usual RGB channel images will just show as a blue square.
So how to go from a normal RGB channel image to a pure alpha-channel. I use Photoshop and there is probably more than one way, but what works best for me is creating a “Gradient map” adjustment layer.
To create a gradient map adjustment layer go to the “Layers” tab and click “Create fill or adjustment layer…” like below.
Afterwards save your image and all should be fine.