i have vista and the easiest way i found is to open the pic in windows photo gallery click the "email" button on the top ribbon, then it will ask what size you want to make it, then an email dialog window opens. right click on the file name (yaddayaddayadd.jpeg) select open, this will open a new photo gallery window with the resized pic, right click on the pic select copy then right click on the desktop select paste and it will be on your desktop in the new size format.
if ya have any questions shoot me a pm