Thanks Vortex! I have experienced this issue before too. I know that it only occurs with the OpenGL renderer and I actually figured out why. When I was writing my own game engine, I noticed that OpenGL requires the width and height of all textures to be powers of 2. Which is obviously what is causing that issue. All of the textures need to have dimensions that are powers of 2 in order for them to work properly with Ogre's OpenGL renderer. For some reason, Direct3D doesn't seem to care...