Using Adobe Photoshop you can create an action to help you in creating a watermark on multiple photos.

To get started you will need a file with the watermark desired and what is very important is to have the watermark on a separate layer and not on the background layer.

With this cleared let us proceed:

1. Open a photo in Photoshop.

2. Navigate to the Actions palette (Window | Actions) and click the Create New Action icon located at the bottom.

3. Give a name to this new action.

4. At this point you need to open the file with the watermark. Once you do this, select the watermark layer and copy it Ctrl - C (Command - C on a Mac).

5. Go to the initial photo and paste what you have just copied using Ctrl - V (Command - V).

6. In the Layers panel lower the opacity of the watermark so that it gets a transparent look. You can use a 50% Opacity.

7. Just a few things and you are set to put a watermark on multiple photos. Click File | Save for Web & Devices and after you adjust the settings save the document.

8. When you are done, close both files and click the Stop icon in the Actions palette to finish recording.

9. From now on you can use this action on individual images to make everything easier. Good look with it!