Maybe all images are not of the correct size, but either the third one is not scaled that much, or OpenGL Anti-aliasing is doing a better job here. Try to use a background with 2^n width and height (not necessarily same n for both) and use only a part from that texture for your screen. that avoids scaling of the images.
That sounds like a driver thing - I had that before, ugly artifacts and colour banding and stuff. Nothing wrong with Irrlicht, you just need to update your drivers.
