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ImageMagick?

windows irfanview, perhaps this will

When talking about PHP — library (supports engines ImageMagick, GraphicsMagick, GD) can be downloaded here code.google.com/p/phpresizer/
If it is about Photoshop — you can record an Action and using batch processing of files (all the same in Photoshop) you can make even far more flexible and advanced automated image processing algorithms.

remember, to ACDSee 7 was some kind of plugin.
maybe newer versions can do is "escalope".

Open photoshop. Click Alt + F9. In the opened window, click on the icon in the bottom row Create New Action, click Record. Next, open any file and perform all the actions that will be repeated every file in the folder (in your case Image -> Image Size). Upon completion of pressure to Stop Playing in the window action, in the bottom row of buttons.
\r
Next:
File -> Automate -> Batch
\r
In the upper part of the choose your recorded action.
In Source menu, choose Folder.
In the Destination menu, choose Save and Close.