Hello,
I’ve had the same problem for ages. I figured out all you had to do was set a width of the portfolio slideshow’s div and then state that all the images have a margin left and right to auto. This code works perfectly for me.
div.slideshow-next {display: none;width:900px;}
div.portfolio-slideshow {width:900px;margin-top:10px;margin-right:auto;margin-left:auto;position: relative;}
div.portfolio-slideshow div.first.slideshow-next{display: block;width:900px;}
div.portfolio-slideshow, div.slideshow-nav, div.slideshow-thumbs {display: none;}
div.portfolio-slideshow img {margin-right:auto;margin-left:auto;}
div.slideshow-nav {padding:0 0 6px 0;color: #999;}
div.slideshow-nav a {color: #999;border-bottom:none;}
div.slideshow-nav a.pause {margin: 0 15px 0 0;color: #999;border-bottom:none;}
div.slideshow-nav a.play{margin: 0 15px 0 0;color: #999;border-bottom:none;}
div.slideshow-nav a.slideshow-prev {margin: 0 10px 0 0;color: #999;border-bottom:none;}
div.slideshow-nav a.slideshow-next {margin: 0 25px 0 10px;color: #999;border-bottom:none;}
div.slideshow-thumbs {margin-top:50px;width:100%;}
div.slideshow-thumbs img{width: 50px;height: 50px;}
div.slideshow-thumbs ul#slides{margin:0 0 0px 0;padding: 0;}
div.slideshow-thumbs li {display: block;float: left;list-style-type: none;padding: 0 10px 10px 0;}
.slides a img.attachment-thumbnail {opacity:0.25;filter:alpha(opacity=25)}
.slides a img.attachment-thumbnail {opacity:0.25;filter:alpha(opacity=25)}
.slides li a.activeSlide img.attachment-thumbnail, .slides li.activeSlide a img.attachment-thumbnail{opacity:1;filter:alpha(opacity=100)}