change TD background using jscript

[lvermeersch]

i want to change the tablecell background using a script
i am new to getElementByID
i tried something like

My table cell TD id is Cell1
Now i want to change the background image
document.getElementByID('Cell1').background= "backG.jpg";
or
document.getElementByID('Cell1').background= url(backG.jpg);

This doesn't seem to work
although i can get the current imagename displayed with
alert(document.getElementByID('Cell1').background);
What am i doing wrong

[lvermeersch]

I can add my table code to make things more clear.

<table width=100% >
<tr> <td width=170px id="MenuafbSM1" background="Clown button.jpg">
<span id=MenutekstSM1 language="javascript" onmouseover="alert(document.getElementById('MenuafbSM1').background);" onclick="document.getElementById('ID_Scherm').contentWindow.document.location='Mijn vrienden_GEN.htm'"> Mijn&nbsp;vrienden</span>
</td>
</tr></table>

I now have used
onmouseover="alert(document.getElementById('MenuafbSM1').background);"
this gives Clown button.jpg as result but notice only the second time i hover over the span, the first time i get Undefined as a result.
Probably this causes to prevent the background cellchange
and i want to use
onmouseover="document.getElementById('MenuafbSM1').background='Clown button2.jpg';"
to change the cell background when hovering over span but this does not work.
(Don't bother about the table span construction, this has to be this way because it is generated code and has allso other functions.)