One feature of Photoshop I wish I had known about when I was just beginning to create for web is Save for Web & Devices. It’s under the Photoshop menu item, then just under Save As. Also the shortcut keys Shift+Command+Option+S (Mac) or Shift+Control+Option+ S (PC) will bring up the Save for Web & Devices window.
It’s a feature I’ve been using for years, and something that’s so ingrained into my work flow I don’t even think twice about it. If you’re saving a image be it a jpeg or png or gif, you should really be using this. It will cut down on file sizes drastically, especially if you’re uploading for use on galleries or creating a image heavy website.