When creating an application the developer must define the target system screen resolution and color depth.  This color depth information is used internally to decide how to create and render display elements in an efficient manner.  When adding images to the user interface it is always preferable to create them in the desired color depth.  If the application will be running in 16bit color then the most efficient image to render will be a 16bit image.  If alpha blending/transparency is not required when this image is rendered then it is advisable to create images in the application color depth or at least remove the alpha channel in the image.