The first step is to put all your original sized images in one folder. Now create another empty folder called 'thumbnails'.

The first step we're going to take is to rename all our images to something generic. In my case, all my photographs are from the zoo, so I am going to use zoo###. To do this you can use the file browser in the top right of the photoshop window. Once you get to the folder with all your images, press ctrl+a to highlight them all. Now we can right-click and select batch rename.

Our next step is to create the action we're going to use on all the photographs. To do this, open one of the photographs; it doesn't matter which one. Now click on the actions tab and select new action as shown here.

This will launch the new action window. In our case, I'm choosing to use thumbnails of width 100. You can make them as big or small as you like, but generally somewhere between 100 and 250 is best. The number entered here doesn't matter, this is just what the action will be called. Storing in the default actions set is fine. Leave the function key and color alone. Hit Record.

The action is now recording every move you make until it is told to stop recording. In our case its quite simple, all we want to do is hit Image --> Image Size and set the width to 100. Make sure the Constrain Propotions box is checked so that the height is also adjust appropriately. You're image should now be 100 pixels wide (or however wide you decided to make it). Now back at the actions tab hit the stop button which is the square beside the red circle. Your action has now been recorded. Close your image and do not save.

Now that we've created the action we're going to use it to perform a batch action. This means that the resize we just performed on the one image we are going to tell photoshop to perform on all our images with just a few quick clicks. To open to batch menu select File --> Automate --> Batch. This should open the window seen above. Now, this seems a bit intimidating at first glance, but we're going to walk through it setting by setting.

In the "Play" frameset we are able to select the action we want to perform on our image. Obviously, we need to select the resize 100 action we just created.

In the "Source" frameset our first task is to tell photoshop which files we want to perform the action on. In our case, the source is going to be the folder that all the images are stored in. So select Folder and then click Choose and browse to the folder where they are stored. For our purposes today leave the 3 checkboxes unchecked.

In the "Destination" frameset we are going to tell photoshop where to store the images after the action has been performed on them. Again, our choice will be Folder and then click Choose and select the 'thumbnails' folder we created at the beginning of this tutorial. Leave the checkbox blank and set up the file naming exactly the same as we did when did the batch rename. This will ensure that the thumbnail name corresponds to the original image name.

You're all set. Now all you have to do is hit OK and let photoshop do the rest. Performing batch actions in photoshop can save you an enormous amount of time, especially if there's five or six hundred images you wanted to resize. Instead of manually doing each on you perform the task on one image, save it as an action, and then batch all of the images. Enjoy!