I think your outgoingSlideEl
is an list element
like li
Then you should use find() to find your image src
like,
var imgSrc = $(outgoingSlideEl).find('img').attr('src');
Full Code
$('.cycle-slideshow').on('cycle-after', function(e, optionHash, outgoingSlideEl,
incomingSlideEl, forwardFlag) {
var imgSrc = $(outgoingSlideEl).find('img').attr('src');
});