Hello wmbuRn,
I was looking into the issue that you reported. I believe that the issue that you are seeing is due to the texture getting up sampled. The reason that a texture that is a power of two may appear sharper is because the engine more adept at handling them. You could try adjusting your maximum Texture Size to the closest power of two that is lower than the current image size. This will cause it to get down sampled first and will help make the image a little sharper. I hope that this information helps.
Make it a great day