Hai provato così?
codice:
.tabella td a:hover {
	text-decoration:none; 
	background:black; 
	color:white; 
	-webkit-border-radius:24px; 
	-moz-border-radius:4px; 
	border-radius:4px;
}
.tabella td img a:hover {
	text-decoration:none;
	-webkit-border-radius:0px; 
	-moz-border-radius:0px; 
	border-radius:0px;
}

/*OPPURE*/

.tabella td a:hover {
	text-decoration:none; 
	background:black; 
	color:white; 
	-webkit-border-radius:24px; 
	-moz-border-radius:4px; 
	border-radius:4px;
}
.tabella td img:hover {
	text-decoration:none;
	-webkit-border-radius:0px; 
	-moz-border-radius:0px; 
	border-radius:0px;
}