RedPhoenix i think it would be understandable if post a screenshoot. I guess you .png image have alpha channel, so you see the alpha blending errors, this is not engine bug its feature of hardware.
Anyway, the manual says:
PNG images behave like TGA images, but are not recommended because they are loaded not by the engine, but by the DirectX library, and are not guaranteed to be automatically restored when the video device gets lost. This means that they can lose their content f.i. when a fullscreen application is minimized.
