Wallpaper position error

I've searched through some of the posts here regarding wallpaper positioning - specifically tiling - but none of them seem to address my issue.

I have 2 VistaHP machines and 2 XPPro machines.

I have a .jpg, which is 363x261 and want to tile it.
On the XP machines, I just select "tile" from the drop-down menu and TADA! - the wallpaper is tiled.
The Vista machines, however, like to put up a fight. BOTH Vista machines do NOTHING when I click ANY of the 3 options - 'tile' 'fit to screen' 'center'.
The wallpaper (no matter how small) gets stretched to fit the entire screen.

Here's the funny thing: I have 2 Vista machines @ home, 1 @ school, and 1 @ work and NONE of them will tile or center! All the Vista machines I work with only did tiling when the Vista install was fresh. But after a few months of use, it stops.

I'm not the only one to encounter this. I've spoken w/ friends and co-workers and they all encounter this.

Please help - I want to tile a picture! Reinstalling Windows is NOT an option. There's gotta be something in the registry that is preventing the 'tile' and 'center' options from working.

Thanks!
AP

see here http://www.vistax64.com/tutorials/91134-desktop-background-wallpaper.html
read poin 5 and 6.by pont 6 look exactly on figure:
by option:How should figure positioned really don't work option 3 ?

How works center options depends also from image size and screen size...

If you set wallpaper by using (for example) Irfan Viewer you have the same effect?

Thanks, that helped... Vista seems quirky and will only 'tile' and 'center' my images when I select them from the "desktop background" box, yet when I right-click to set as bkg and then click 'tile' or 'center' it won't do it.
Anyway, thanks for the help!