Image resizing is not present in the default picture and fax viewer of windows Vista/Xp so people have to look for alternatives, earlier in XP this could easily be done using Image resize Power toy but vista has no such toy for it so Sourceforge comes to rescue and here is a small software that will add the functionality to resize single or multiple images from right inside Vista/Xp explorer using right click  .

The source website is http://imgresizer.sourceforge.net/

you can directly downlad it from Vista image  Resizer ( resize-o-matic )