Command-line to open and save a file?

Hi,
I was wondering if there was a command-line to open a file, and also one to save it. I’m trying to figure out how to write scripts so i can open and save multiple files. I don’t want to rename, I just want to open and save them under the same name. If you’re curious why, it’s because my friend found that simply opening and saving a picture on a computer that has been imported from a camera drastically reduces the file size because computers can compress better i guess. i know nothing about scripting and would greatly appreciate help if anybody knows how to do this.

p.s. that is a real e-mail address