You can even leave the line below if you want the element also to have a background color:
document.getElementById('bcen').style.backgroundColor='black';
But also add:
document.getElementById('bcen').style.backgroundImage='url(CAMINHO DA IMAGEM)';
document.getElementById('bcen').style.backgroundSize='cover';
The style.backgroundImage
will insert a background image and style.backgroundSize='cover';
will cause the background image to occupy the entire element.
Example:
<!DOCTYPE html>
<html>
<head>
<title>Page Title</title>
</head>
<body>
<button id="bcen" onclick="if(--this.textContent<=0) alert('You\'ve won!');" value="10">10</button>
<script>
document.getElementById("bcen").style.width =
"300px"; //Size
document.getElementById("bcen").style.height =
"300px"; //Size
document.getElementById('bcen').style.fontSize="100px"; //size
document.getElementById('bcen').style.color = 'white'
document.getElementById('bcen').style.backgroundColor='black';
document.getElementById('bcen').style.backgroundImage='url(https://i.ebayimg.com/00/s/MzAwWDMwMA==/z/~z8AAOSwEK9UIcNC/$_35.JPG?set_id=2)';
document.getElementById('bcen').style.backgroundSize='cover';
document.getElementById('bcen').style.borderRadius = "50%";
document.getElementById('bcen').style.outline = "none";
</script/>
</body>
</html>