Open the image in preview and go to Tools > Adjust Size in the menubar:
In the resulting dropdown window, change the pixels to the largest size you’ll ever need the image to be in your website. For instance, if you’re resizing the header banner and you know your theme is only 900px wide, then there is no need for your header banner image to be longer than 900px. Resize it so that it is only 900px. Make sure you have the “scale proportionally” checkbox checked. The resulting size section of the window will show you how much space you’re saving in KB.