Create a php block.
Put the following code in it, filling in the urls to your banners and links as needed, one per line:
$banners = array (
'<a href="http://link/to/site" target="_blank"><img src="http://url/to/img"></a>',
'<a href="http://link/to/site" target="_blank"><img src="http://url/to/img"></a>',
'<a href="http://link/to/site" target="_blank"><img src="http://url/to/img"></a>',
'<a href="http://link/to/site" target="_blank"><img src="http://url/to/img"></a>',
'<a href="http://link/to/site" target="_blank"><img src="http://url/to/img"></a>',
'<a href="http://link/to/site" target="_blank"><img src="http://url/to/img"></a>',
'<a href="http://link/to/site" target="_blank"><img src="http://url/to/img"></a>',
'<a href="http://link/to/site" target="_blank"><img src="http://url/to/img"></a>',
'<a href="http://link/to/site" target="_blank"><img src="http://url/to/img"></a>',
'<a href="http://link/to/site" target="_blank"><img src="http://url/to/img"></a>',
'<a href="http://link/to/site" target="_blank"><img src="http://url/to/img"></a>'
);
shuffle($banners);
echo $banners[0];
It's easy to edit, you don't have to have any external files, and you can have as many or as few banners as you want.