Zwei Versuche habe ich bereits unternommen:
<form method="POST">
<p><input border="0" src="bild1.gif" name="submit" type="image"
onmouseover="submit.src='bild2.gif'" onmouseout="submit.src='bild1.gif'"></p>
</form>
---
<form method="POST">
<p><a onmouseover="document['fpAnimswapImgFP1'].imgRolln=document['fpAnimswapImgFP1'].src;document['fpAnimswapImgFP1'].src=document['fpAnimswapImgFP1'].lowsrc;"
onmouseout="document['fpAnimswapImgFP1'].src=document['fpAnimswapImgFP1'].imgRolln" href="javascript:void(0)">
<input border="0" src="bild1.gif" name="fpAnimswapImgFP1" type="image" id="fpAnimswapImgFP1" dynamicanimation="fpAnimswapImgFP1" lowsrc="bild2.gif"></a></p>
</form>