Writing callback functions

I'm using jQuery to write things like this:

    $('#current_image').fadeOut(function(){
        $('#current_image').attr('src',newImage).show();
    });

This is lovely, once the fadeOut has completed, the nested bit executes.

I'd like to make my own function here that would replace fadeOut. What would my function look like so that this code would work?

    $('#current_image').customFadeOut(function(){
        $('#current_image').attr('src',newImage).show();
    });