You missed to close your img tag:
Code:
<a class="thumbnail" href="#thumb"><img src="tabs/narrowboatboxes/test1.jpg" width="100px" height="66px" border="0" style="opacity:0.4; filter:alpha(opacity=40)"
onmouseOver="this.style.opacity=1; this.filters.alpha.opacity=100"
onmouseOut="this.style.opacity=0.4; this.filters.alpha.opacity=40"/>
<span><img src="tabs/narrowboatboxes/test1.jpg"></span></a>
<a class="thumbnail" href="#thumb"><img src="tabs/narrowboatboxes/test2.jpg" width="100px" height="66px" border="0" style="opacity:0.4; filter:alpha(opacity=40)"
onmouseOver="this.style.opacity=1; this.filters.alpha.opacity=100"
onmouseOut="this.style.opacity=0.4; this.filters.alpha.opacity=40"/>
<span><img src="tabs/narrowboatboxes/test2.jpg"></span></a>
<br/>
<a class="thumbnail" href="#thumb"><img src="tabs/narrowboatboxes/test3.jpg" width="100px" height="66px" border="0" style="opacity:0.4; filter:alpha(opacity=40)"
onmouseOver="this.style.opacity=1; this.filters.alpha.opacity=100"
onmouseOut="this.style.opacity=0.4; this.filters.alpha.opacity=40"/>
<span><img src="tabs/narrowboatboxes/test3.jpg"></span></a>
<a class="thumbnail" href="#thumb"><img src="tabs/narrowboatboxes/test4.jpg" width="100px" height="66px" border="0" style="opacity:0.4; filter:alpha(opacity=40)"
onmouseOver="this.style.opacity=1; this.filters.alpha.opacity=100"
onmouseOut="this.style.opacity=0.4; this.filters.alpha.opacity=40"/>
<span><img src="tabs/narrowboatboxes/test4.jpg"></span></a>
See if adding highlighted helps.
Bookmarks