Here I show three examples of a graphic and compare the image statistics for each. I started with the first, and optimized in two different formats.

Note that you can further optimize a web graphic for loading through resizing the original down to a smaller size, or by using a thumbnail of the original, so that folks can choose whether or not to wait and see what the graphic looks like. I've used the thumbnail structure in a few pages in this website for that reason.

It's a good idea to try saving a weighty graphic or photo in more than one compression format and then comparing the loading times. In this example, the 50% JPG compression won for speed of loading.