@Paul,

Some times there a few adjustments that are needed depending on the theme that you are using and how well coded it is. I couldn’t look at it since your gallery is password protected could you please provide a link to a test gallery.

The problems is that your thumbnails are all different sizes, add this code to your theme’s styles.css file. Note this this may have problems with older browsers.

div#ims-mainbox .ims-img {
display: -moz-inline-box; 
display: inline-block;
vertical-align: middle;
float: none;
}