Instructions
Launch Adobe Photoshop CS4. Click the "File" menu and select "Open." Select one image from the batch that you want to add a watermark to. Click "OK" in the bottom right corner and the picture will open up in the editor.
Click the "Actions" button near the top of the right margin and a new actions palette will open.
Click "Create New Action" and a new window will open up. Add "Watermark" to the "Name" field to name your action. In the "Function Key" field select a keyboard shortcut. In the "Color" field, select the color for the text you will be using. Click the "Record" button.
In the new window that opens, create the watermark for your action, click "OK" in the top right of the window and click the "Stop recording" button in the bottom of the actions palette.
Open the multiple images you want to add the watermark to and press the keyboard shortcut you created. The action will add the watermark to all of the open images.