jQuery Change Image src with Fade Effect

You have to make it fadeOut() first, or hide it.

Try this :

$(".thumbs a").click(function(e) {
    e.preventDefault();
    $imgURL = $(this).attr("href");
    $(".boat_listing .mainGallery")
        .fadeOut(400, function() {
            $(".boat_listing .mainGallery").attr('src',$imgURL);
        })
        .fadeIn(400);
});

It should fadeOut the image, then change the src when it’s hidden, and then fadeIn.

Here’s a jsFiddle example.

Edit: you can find a more recent & better solution in Sandeep Pal’s anwser