Exactly, its a great time saver. In case you dont know how to do it, or in case it helps others I am gonna write up a quick tutorial on how to do it.
Update:
Quick and Dirty Actions Tutorial:
Step 1
Fire up Photoshop and load up a single image you wish to add a border too. Depending on your layout, you may need to reset your workspace. (Window>Workspace>Default Workspace) This will reset all palette locations and which palettes are currently visable.
Step 2
Find the Actions Palette Tab and Click on it.
Step 3
Click on the New Set button, a dialoge will pop up asking for a name. I typed in "Custom Actions" But you can use whatever you want. This is just to help organize
your actions. Keeping them seperate from the stock Adobe ones.
Step 4
After you create your new set, you want to create the actual action. So click on the New Action button, another dialoge box will pop up.
You can pretty much leave all the settings the way they are, except you will want to give it a name, so you can easily identify it. I chose "Add Border" for a name, since thats what this action will be doing.
After you click record photoshop will then be recording everything you do. So you can either hit the black stop square in the actions palette to catch your breath before you want it to record, or you can continue on.
Step 5
So now we want to add the border to our image
while the action recording is ON. There are multiple ways to do this, but I will just demostrate the fastest and most simple method.
Go to Edit>Stroke
A dialog will pop up, from here you can change the size of your border (stroke) and the color. Near the bottom you can choose if you want the border to created centered on the edges (or if you had a selection off that marque) all pixes outside or inside of it. At this point just leave those settings the way they are.
I chose a 10 pixel black border, but you might want to do more or less or a different color, its up to you.
Step 6
Now, all we need to do is save the document and close the document window. Do both of these "actions" while you still have record on. When saving the document be carful not to overwrite the originals.
When you are done just hit the stop button in the actions palette. Then from here you can just load up all 50 of your images into photoshop and just hit the "Play" button for your action you just created and it will add the border, save and then close it. So just do that for all 50 images, there is a batch function for stuff like this but thats outside the scope of this tutorial.