<img src="buttonup.gif" width="100" height="50" name="rolloverup"
onMouseOver="document.images[0].src='buttonover.gif';"
onMouseDown="document.images[0].src='buttondown.gif';"
onMouseUp="document.images[0].src='buttonover.gif';"
onMouseOut="document.images[0].src='buttonup.gif';">
<img id="rollover" src="buttonup.gif" width="100" height="50" name="rolloverup"
onMouseOver="document.all.rollover.src='buttonover.gif';"
onMouseDown="document.all.rollover.src='buttondown.gif';"
onMouseUp="document.all.rollover.src='buttonover.gif';"
onMouseOut="document.all.rollover.src='buttonup.gif';">
<img src="buttonup.gif" width="100" height="50" name="rolloverup"
onMouseOver="this.src='buttonover.gif';"
onMouseDown="this.src='buttondown.gif';"
onMouseUp="this.src='buttonover.gif';"
onMouseOut="this.src='buttonup.gif';">
<img src="buttonup.gif" width="100" height="50" name="image1"
onmouseover="image1.src='buttonover.gif';"
onmouseout="image1.src='buttonup.gif';">
<img src="buttonup.gif" width="100" height="50"
onMouseOver="t=this.src;this.src='buttonover.gif'"
onMouseOut="this.src=t">

Macromedia Dreamweaver - SwapImage/Restore Behaviour (Requires Script in Head of document)

<img src="buttonup.gif" width="100" height="50" name="dwrollover"
onMouseOut="MM_swapImgRestore()"
onMouseDown="MM_swapImage('dwrollover','','buttondown.gif',1)"
onMouseUp="MM_swapImage('dwrollover','','buttonover.gif',1)"
onMouseOver="MM_swapImage('dwrollover','','buttonover.gif',1)">