• Open the image and rotate it as necessary
• Resize the image
  • Click Image→Image Size…
  • Enter a width of 500 pixels, the height should change automatically.
  • Click OK
• Save for Web
  • File→Save for web…
  • Choose “JPEG” and “Maximum” from the dropdown options.
  • Click OK and save the new image in a place where you can easily retrieve it for uploading.

If you can't get your hands on a copy of photoshop, this software can stand-in for most uses and it's free… http://gimp-win.sourceforge.net/stable.html

TODO