I help maintain a site for a craft business. One of the challenges is multiple sizes of images for new stock each year. I used to go through and resize each image manually with a program like Gimp.
I finally got smart and installed ImageMagick.
With a Rakefile, I can pass each image into the convert command with the proper sizing, and get the output into a new folder, ready for upload. The code is shown here:
One catch to keep in mind if you work on a Mac. There can be orientation metadata from some images that OS X will honor, but a browser will not. If your images show up on the web in the wrong orientation, look into ImageMagick convert command’s -rotate option.