An image should always be a substantial contribution to the content, it should convey some information. An image simply for decoration (or, even worse, for your ego!) is just a waste of bandwidth.

A typical example of how you can use images communicatively is a pair of images with the captions "Before" and "After" (or "With" and "Without"). In the text you are describing any effect resulting from using your product. In the following you are presenting a pair of images "Before" and "After" using your product. Hardly anything will be more convincing!

To do that, however, it will be useful to present both images side by side. So the visitors can compare them exactly, on their own, free of any time pressure. (Isn't just that what makes the Internet a wonderful communication medium?) To that end you could, for example, present the images in a table:

      <TABLE WIDTH="..." ... ><TR>
      <TD WIDTH="..." ... ><IMG SRC="..." ALT="..." WIDTH="..." HEIGHT="..."></TD>
      <TD WIDTH="..." ... ><IMG SRC="..." ALT="..." WIDTH="..." HEIGHT="..."></TD>
      </TR></TABLE>