..Disclaimer...if your a MAC user...you're on your own.
It's very easy on a Mac. Just double click the pic file and it automatically opens in preview mode. Duplicate the pic to make it editable, and then resize or crop the pic to whatever size you want. I always set the width at 800 pixels, and it displays correctly on the screen, and is usually less than a meg. Mac's also let you fine tune the file size for downloads.
As a rule, 3 letter extensions are more apt to display correctly. filename.jpg will always display, but filename.jpeg may not.
