1. At first imagine you want to create a photo gallery for the web. You want each photo to be 300 * 225 pixel and not bigger then 15 kb. O. K. of course you will say: "Oh man, shut up! I don't need a tutorial how to modify a picture". And you're right. But now imagine you don't want to modify one picture but about 100. :-)
2. O. K. This is my part.
3. At first we have to prepare our work. Do all the pictures in one folder.
4. Now open one picture in Photoshop.
5. Watch at my picture. Open your action-palettes and create a new folder. Call 'em Picture maker.
6. Open this folder and create furthermore an actions.
7. Now we begin to make our actions. Begin recording.
8. Go to Image-Image Size and modify your picture.
9. Now go to file-save for web. Save your image in order not to become bigger then 15 kb.
10. Press the stop button in your actions-palette and your first action is ready. Congrats. Imagine what time you save with actions.:-)
11. Now we want to automate this action.
12. For that reason go to file-automate-batch
13. Use this settings. Note: Source is the folder with your original files. Destination is the folder you wish to place your modified pictures in.
14. Press O. K., lean back and save hours of expensive time. Enjoy.
More Tutorials: 
