You can do this with javascript. This way is messy, but it's inline so that you don't have too much trouble if you don't know what javascript is.
Code:
<img src="http://tbn3.google.com/images?q=tbn:flrjig-qfoXt_M:http://www.blackstardojo.org/shop/images/Pokeball.gif" onclick="if(this.src=='http://tbn3.google.com/images?q=tbn:flrjig-qfoXt_M:http://www.blackstardojo.org/shop/images/Pokeball.gif') { this.src = 'http://tbn0.google.com/images?q=tbn:jQFsbERt4slGyM:http://homepage3.nifty.com/kuma~s-world/pokemonicon/drowzee.gif'; } else { this.src = 'http://tbn3.google.com/images?q=tbn:flrjig-qfoXt_M:http://www.blackstardojo.org/shop/images/Pokeball.gif'; }">
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