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<<click [img[testimage1]] >> <<replace "#test">>test text<</replace>> <</click>>To this?:
<<click <img src="testimage1" onmouseover="this.src='testimage2'" onmouseout="this.src='testimage1'"/> >> <<replace "#test">>test text<</replace>> <</click>>
First you need to identify the img element, and you can do that by wrapping the <<click>> macro in a span with a class like so.
Next you need to reference the image in your CSS, which is easy because SugarCube's image markup links work there as well.
Obviously you should change the .someclassname class name used in the above with a more meaningful one.