[VIPStephan]

Almost.
The basic syntax looks like this:

$('#tag-hover').mouseenter(mouseInFunction).mouseleave(mouseOutFunction);

And your code specifically would look like this then:

$('#tag-hover')
	.mouseenter(
		function(){
    			$('div.tag-font').stop(true, true).fadeOut(100, function(){
    				$('div.tag-image').stop(true, true).fadeIn(400);
    			});
    		}
	)
	.mouseleave(
		function() {
    			$('div.tag-image').fadeOut(400, function(){
    				$('div.tag-font').fadeIn(100);						 
    			});
    		}
    	);

This is a very verbose way of writing it, it could as well look like this:

$('#tag-hover')
	.mouseenter(function(){
    		$('div.tag-font').stop(true, true).fadeOut(100, function(){
    			$('div.tag-image').stop(true, true).fadeIn(400);
    		});
    	})
	.mouseleave(function() {
    		$('div.tag-image').fadeOut(400, function(){
    			$('div.tag-font').fadeIn(100);						 
    		});
    	});