I am trying to make a link menu but the list items are stacking up vertically rather than horizontally.

Can anyone see anything wrong? The list is being used in a Wordpress post.

<div class="image-holder">
<div class="image"><img src="http://ecx.images-amazon.com/images/I/41XZTVKkN6L._SL500_AA280_.jpg" width=200 ></div>
<ul><li><a href="#">one</a></li><li><a href="#">two</a></li></ul>
</div>

}
.image-holder {
    float:right;
    width:202px;
    height:242px;
    position:relative;
    padding-left:5px;
}
.image-holder ul {
    text-align: center;
    float: left;
    list-style: none;
    padding: 0px;
    margin: 1px;
    border: 1px solid #000000;
    width: 90px;
    font-weight:bold;
    display:block;
}
.image-holder ul a {
    text-align: center;
    color:#000000;
    text-decoration:none;
    background-color:#96D2F3;
    display:block;
    }
.image-holder  ul a:hover {
    text-align: center;
    color:#000000;
    text-decoration:none;
    background-color:#00FF00;
    display:block;
    }
.image {
    float:right;
    border:#999999 1px solid;
    width:200px;
    height:200px;
    position:relative;
    }

Many thanks.

display:block; means that something will act like a block level element and be stacked like a <div> or a <p>.

You have <ul> floated left but that does not make its' <li>'s float left.

You have neither floated the list items nor used displayed "inline" so the default is stacked.

I suggest you look here and find the menu you want to achieve:
http://css.maxdesign.com.au/listutorial/