<a href="#" onmouseover="image1.src='./X.JPG';" onmouseout="image1.src='./leer.JPG';"><img name="image1" src="./leer.JPG" border="0"></a>
<a href="#" onmouseover="image1.src='./X.JPG';" onmouseout="image1.src='./leer.JPG';"><img name="image1" src="./leer.JPG" border="0"></a>
Versuch doch mal als "name" einmal "image1" und nochmal "image2". :)