Auto Fixed Width Images

Hello new here. I’ve searched the forums endlessly, maybe I am searching with the wrong keywords, so I apologize in advanced if there is a thread that covers it. Please direct me to the appropriate thread if it exists thanks.

http://thecreepster.com
I am trying to make the images that I post automatically be fixed at 500 wide no matter what the size I upload the photos at. Currently I have to manually type the width and figure out the height after the fact.

Is there a way to hack it so when I upload the images it automatically makes it 500 wide and figures out the height as well?

Thanks in advance!