There is a new PowerToy type add-on for Windows Vista users. The Image Resizer PowerToy Clone. It is a free open source Windows Shell Extension that enables you to resize one or more image files on the fly by simply right-clicking on them.
This enables easy image resizing on virtually any version of Windows. So far I have tested all common image types (png,jpg,gif,bmp) and it resized them without issues. Download it from here for free.