I solved it by hiding all other images except for the first image, instead of using position:absolute.
So My CSS is now:
#slideshow_thumb {
width:22%;
float:left;
margin-bottom:0;
padding-bottom:0;
margin-right:3%;
}
#slideshow_thumb img {
display:none;
width:100%;
height:auto;
}
#slideshow_thumb a:first-of-type img{
display:block;
}