Image Resizer, a Microsoft Powertoy, creates a new option in the right click menu, to resize one or more images immediately. There appears a dialogue to let you choose between some standard sizes and (advanced) let you overwrite the existing files instead of creating new ones. Excellent, if you don’t want much control over the resize parameters.
MS Image Resizer does not work on Vista or Seven. If you use one of these later Windows versions, try a clone of the Image Resizer Powertoy for Windows XP – a PowerToy that allows you to right-click on one or more image files in Windows Explorer to resize them. This clone was created to extend support to non-XP and 64-bit versions of Windows (including Windows 7 & Vista). I have tried it, and it works just fine.