It's a CSS property:
Code:
<style type="text/css">
element { display:none; }
</style>
If you want to modify that using Javascript, you should be able to access it in the following way:
Code:
<script type="text/javascript">
element.style.display = 'none'; //hide it
element.style.display = 'block'; //display as a "block" element, like a <div>
element.style.display = 'inline'; //display as an "inline" element, like a <span> or normal <img>
</script>
Of course you'll need to put all of this together using functions and conditionals (ifs), but the tools above are the basics for what you'll need to do. If you alternate displaying one image instead of the other, then you'll be able to effectively "change" the image onclick to the other, and that way keep the same code as you have now for the onmouseover and other code.
Note that you can use this format if you need two or more element in an html event attribute:
<img onclick="function1(); function2();">
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