Scale all the chosen images into 800 pixels wide

Scale the chosen images to 800 pixels wide without changing aspect ratio.

Directly running the code, the user could specify multiple images and these images will be automatically scaled to 800 pixels wide without changing aspect ratio.

It's very convenient for those who want to poster their shot images online while the album website puts a size limit such as 600KB. Scaling the images is therefore necessary and this code could directly help people accomplish such a task.