Picture resizing is a common task that users and administrators (Especially webmasters) have to deal with. Commercial applications such as Photoshop can perform this task flawlessly however they have high cost and excessive functionality.
VSO Image Resizer is a free utility that can resize images and convert them in various formats. The products homepage offers a good description of all capabilities that software has. Plus it can convert and resize a batch of images simultaneously. It is very simple to use VSO Image Resizer. Still if you are lost please read this simple VSO Image Resizer HOWTO.
Make sure you take a look at VSO IR if you need a simple picture manipulation tool.