Photo Magician is a fast and easy to use batch image conversion tool that can resize or convert the format of a folder full of digital photos with the minimum of hassle.
At its simplest you might just select your input and destination folders, then choose a profile that defines what you want to do. There are built-in profiles for many common sizes (320x240, 640x480, 800x600, iPod/ iPhone/ iPad, PSP, Zune and more), and these may be all you need.
Alternatively, you can choose a custom destination pixel size for your images, or a percentage scaling factor. And the program can also convert all the images it finds to the same format (JPG, PNG, GIF, TIF and BMP output is supported).
Some useful conversion settings allow you to define exactly how the program will work. So you can include EXIF data or not, exclude images below a defined resolution or file size, or preserve or overwrite the original files, for instance.
And if you want to automate the process then it might be worth trying the program's command line options, which allow you to control most of its features from your own scripts.