Make a JavaScript/jQuery slideshow
70 68(JS) + 22(CSS) characters

setInterval('$(".s").append($(".s :first").hide().fadeIn(1e3))',5e3)


CSS:

img{position:absolute}


jsfiddle: http://jsfiddle.net/QACAY/

pure jQuery. The only browser-stuff is setInterval, which dates as far back as IE4. jQuery support doesn't reach that far. Javascript syntax here is ancient as well.

Of course, in practice, do not pass strings to setInterval! It is ten characters shorter (fewer with es6 arrow functions), but it's unreadable (madness? This is code golf!), it forces you into the global scope (which we don't mind here), it's slower (but we are far from being concerned with performance of calling jQuery code) and it won't pass any decent code review.

Works best if all images have the same shape. But, it will work just as fine with semitransparent images as long as visible stacking counts as fine and no more background is visible when any single image is removed. 

Since this does not wait for DOM ready, there's a tiny chance that only some of the image elements inside a gallery are loaded when an interval fires. In this case, the stack of images will get reordered. I don't think it's a problem. In fact, I kinda like it. 

After slight tweaking, it looks like code that could easily pass through a code review and get into production (also, be sure to modify the CSS to not be too aggressive):

$(function(){
  $(".s").each(function(_, s){
    setInterval(function(){
      $(":first", s).hide().appendTo(s).fadeIn(1000);
    }, 5000);
  });
});