Wallpaper question/problem

My "problem" is simple - if a picture is bigger than the desktop resolution, I want it to "fill" the Desktop (that is, not to use the entire desktop, but keep the ratio), however, if it is smaller, I want it in the "center" (that is, NOT tiled, NOT stretched, just the picture itself). I can't believe that this is not possible. What do you people do when you have both kinds of pictures in a directory and you use them for desktop backgrounds? Tell me there is some way to do that, please. :)

This is possible by going to:

Control Panel > Personalisatioin > Desktop background

However, I have not found a way to do this if you are using the slideshow feature.

Yes, this is exactly my point. In Desktop Background I can change "Picture Position", I know that. I want a mixture of "Fill" and "Center" - Fill when the picture is bigger than the Desktop resolution, Center otherwise.