I have a situation where I need to re-size a lot of images in a folder by a certain %. All the images are different heights and widths so I cannot just say 400 x 300 reduced by 10% is now "X by Y". It's also very inefficient to go through each one and do the math of reducing each one by 10%. 

Is there a way to do this from the command line in Windows? 

**A one liner would be my ideal solution**, but if that's not possible maybe a batch file? 

If that's still not possible I will take recommendations for **free** software but it must be able to reduce image height and width by a percentage. As stated before the images are all different sizes so I do not know the exact dimensions I'm re-sizing them to. I just need them about 10% - 20% smaller. So a tool that will mass re-size to say "200 x 400" will not help me.

All of my research so far has turned up nothing.