Creating an Action
Open up one of the initial pictures, open up the actions palette, click the create new action button which looks a lot like the create new layer button. Name your action, like "Insert Logo" or something. Hit the red circle record button. Insert your logo, resize it, whatever it needs done to it. Save the file. Close the file. Hit the stop recording action button. Go to File/Batch/Automation. Choose a source folder, choose the action you made to apply to the files in that folder, choose a destination folder to save the new images into. Hit okay or go or whatever it says.
On a cautionary note, copy the folder containing your original pics just in case you mess up and run the action to save over the originals.

Action Insurance Policy
Have you ever written an action, and after it's done, you wish you hadn't run it in the first place? Maybe the effect just doesn't look right on the image, or there's a mistake or missing step in your action? Well, here's a tip that will help you, not just when you're testing your action, but even after you've perfected it. Bring up the Actions palette (found under the Window menu), click on the Create New Action icon, and once you're recording, make the first step of your action creating a snapshot. To do this, just open the History palette (under the Window menu) and click on the Create New Snapshot icon at the bottom of the palette. That way, if after the action runs, if you don't like the results you can just click on the saved snapshot in the History palette and the image will instantly return to how it looked before you ran the action.

Troubleshooting Actions? Slow Down!
If you're an advanced user, chances are you're no stranger to using actions, and in fact, you probably create your own (rather than using the default actions that ship with photoshop, many of which redefine the term "useless"). If you do create your own actions, you've already found that you spend more time troubleshooting your actions than you do creating them in the first place. Well, this little tip makes the troubleshooting process a lot easier, and saves you both time and frustration. The problem is (and this won't sound like a problem) Photoshop runs actions so quickly that you don't see each step, or each dialog, so tracking down a missing or wrong step is just about impossible. Luckily, you can actually slow down your action, or even put a pause between each step, by using Photoshop's Playback Options dialog found in the Actions palette's flyout menu. When it appears, you can choose to play your action Step by Step, seeing everything as it happens, or you can choose to enter the number of seconds you'd like it to pause. Then, when you replay the action, you can see everything step by step and track down the culprit.

Actions Power Tip: Add An Action To Your Action
Here's an actions power tip: Did you know that you can build an action that will include an existing action? Here's how it's done: As you're recording your action, just go to the Actions palette, click on the existing action you want to include in your current action, and click the Play button at the bottom of the Actions palette. The existing action will now be added as a step in your current action (pretty scary stuff).

Got A Folder Full Of Images For The Web? Batch 'Em!
Do you have a whole folder of images that you're going to convert to Web graphics? If the images are somewhat similar, don't do them one at a time-automate the process using actions. Start by opening one image from the folder. Go to the Actions palette (under the Window menu) and click on the Create New Action button. Give this action a name (something like Optimize as JPEGs) then go about the business of optimizing this one graphic into a JPEG for the Web. When you're done optimizing it, click the Stop button at the bottom of the Actions palette. Then go under the File menu, under Automate, and choose Batch. In this dialog, under Play, choose the name of the new action you just created. Under Source, choose the folder of images you want converted using that action, and under Destination, choose what you want to happen to those images after they're converted. Click OK, and Photoshop will convert that folder with absolutely blinding speed. This one tip can really change the way you work, especially if you create for print first, then repurpose for the Web afterward.

hey, i have an action to resize and add a sig for the web but i want to edit the text within the action to change the year to 2009 but it won't let me. how do i do that?

A little late on this response :embarrassed:
Did you figure it out? Post azip file with your action and I'll look at it.

For those, I'll create a separate file with a transparent background and the text/image/whatever on the 1st layer. If it needed to change, all I have to do is open the file up and make the change.

Essentially, when batching, since it opens 1 file at a time, the action looks like:

*Open batch image* (Automatically, not in action)
*do whatever you want to the image before adding sig... should be last part of the action.

Open File: Signature (opens in foreground)
Select first layer (not the background)
Select All
Copy
Close (closes sig file)
Paste (pastes into batch image)
Move (usually with a predefined location that is the same number of pixels from 2 edges... bottom and right edge)

I've done it both ways... a separate file with my text, or have the action add the text. Either works.