[midnz]

Maybe you could work with a BBCode I made for my own forum today. I wanted to resize images to 200 x 150 and increase to 800 x 600 with onclick and reduce back to 200 x 150 again with ondblclick. It works well for me.

<img src="http://www.{param}" width="200" height="150" onclick="this.src='http://www.{param}';this.height=600;this.width=800" ondblclick="this.src='http://www.{param}';this.height=150;this.width=200"><!{param}>

I haven't tested this edited version but if it doesn't work the way you want you could play around with it a little:
You must remove the http://www. from any image link for it to work though.

<img src="http://www.{param}" width="800" height="600" onclick="this.src='http://www.{param}';this.height=;this.width=" ondblclick="this.src='http://www.{param}';this.height=800;this.width=600"><!{param}>