javascript - Input Button v. Image -

I have put this in a fiddle to show the results using the input and the results It is simple to present.

  & lt; Input type = "button" value = "get value" on ... click   = "javascript: add ();" & Gt; & Lt; Br / & gt; & Lt; Input type = "image" src = "http://us.cdn2.123rf.com/168nwm/sqback/sqback0904/sqback090400053/4618216-black-button-isolated-on-white.jpg" onclick = "javascript: gt" And, (add);   
 Why is it that I run the Type = "Button" script and display results in the target input, but if instead I use Type = "Image" (in Bella This is the button) runs the script, displays the answer, and then clears the form?  
 Why in one case it stops on display, and on the other it continues to help clean up the answer?  
 
 
 
  type = button  does not submit the form  type = image  is based on