How to automatic change image size?

[toplisek]

How to automatic change image DPI with Photoshop?
What is the best optimimum for WEB publishing for JPG, GIF, PNG?

[RTW1]

select from the menu Save for web and devices you then pick the compresion etc that you want, if happy you can then make an action to do just what you have done for repeat use.
jpeg for most work with images , gif for simple graphics