First you need to hide all the other .slideshow img
elements not being shown when your page first loads. You can do that multiple ways, but here's an example:
$(".slideshow img:not(:nth-child(" + count + "))").hide();
Next, you need to hide the current showing slide when going to the previous one:
$(".slideshow :nth-child(" + count + ")").fadeOut();
Finally, you need to set the count to the number of elements in .slideshow img
when going to the last image:
count = $(".slideshow img").length;
Here's a working example: http://jsfiddle.net/LWBJG/22/