Sometimes, you will have a set of images and wish to do some operation on all of them. This operation might be things like creating thumbnails, putting a border on every image, saving as a different format, or many other operations. Two excellent, but very different ways of doing any of these things, are discussed here: using Photoshop’s ‘Actions’ capability and writing code in a programming or scripting language, often interacting with ImageMagick’s command line tools.
Although not always true depending on what you want to do, note that for many of these batch processing operations, you will have a much easier time if all of your starting set of images are the same size (that is if one is 600×400 pixels, they are all 600×400).
Many people do not know about Photoshop’s batch processing capability via ‘Actions’. Basically, this functionality allows you to record a series of operations you do on a single image and then apply that same series to a large set of images. You can even download a large set of pre-defined actions which other Photoshop users have written.
Linked here are two tutorials with more details on using Actions in Photoshop.
Perl (or another computer language) combined with ImageMagick
By using a programming language combined with ImageMagick, you gain access to the ability to do very complex and very flexible operations on sets of images. However, this does require a programming background and a good understanding of the suite of ImageMagick tools.