title: How to control the size of images and graphics?
body: <p>I've been having trouble arranging graphic elements correctly. Here's an example:</p>
  
  <pre><code>im1 = Import["http://upload.wikimedia.org/wikipedia/commons/5/5c/London_2010_Tower_Bridge.jpg"];
  GraphicsRow[{im1, ImageAdd[im1, Graphics[Disk[]]], Graphics[Disk[]]}]
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/3xGEU.png" alt="example of problems"></p>
  
  <p>The circles are both specified in the same way, but show at different scales, so the diagram that attempts to show how it works isn't very satisfactory. Obviously one is scaled to match the image, but I can't see why they don't appear the same size. <code>Row</code> is similar to <code>GraphicsRow</code> but gives me the same problem.</p>