function mark(ch) 
{
	// do we need to mark?
	if(ch!=document.markch) 
	{
	//make new css
		ch.className='markimage_table';
	// make if ch is marked do back
	if(document.markch) document.markch.className='image_table';
	// put it into markch
		document.markch=ch;
	}
}