En tu CSS añade:
.img-enlace:hover {
cursor: pointer;
opacity: 0.5;
filter: alpha(opacity=50); /* Para que la opacidad funcione en Internet Explorer */
transition: 1.5s;
-webkit-transition: 1.5s;
-moz-transition: 1.5s;
}
que quedaría así:
[COLOR=#333333]* {text-indent:0px; margin:0px; padding:0px; border:0px;}[/COLOR]
[COLOR=#333333]p {font-family:Comic Sans, cursive; font-size:1.0em;color:#FFF68F;text-align:justify}[/COLOR]
[COLOR=#333333]h1 {font-family:Comic Sans, cursive; font-size:1.1em; color:#FFD700; font-weight:bold; text-decoration:none;[/COLOR]
[COLOR=#333333]text-align:justify;}[/COLOR]
[COLOR=#333333]h2 {font-family:Comic Sans, cursive;font-size:1.0em; color:#00ff00; text-align:justify}[/COLOR]
[COLOR=#333333]h3 {font-family:Comic Sans, cursive;font-size:1.0em; color:#00ff00; text-align:justify}[/COLOR]
[COLOR=#333333]h4 {font-family:Comic Sans, cursive;font-size:1.0em; color:#00ff00; text-align:justify}[/COLOR]
[COLOR=#333333]body {text-align:center;background-color:black;}[/COLOR]
[COLOR=#333333]#global {width:1000px; margin:4px auto;}[/COLOR]
[COLOR=#333333]#cabecera {background-image:url(objetos/fondo.png); height:85px;}[/COLOR]
[COLOR=#333333]#logotipo { width:290px;float:right;}[/COLOR]
[COLOR=#333333]#publicidad {font-family:Comic Sans, cursive;font-size:1.5 em;color:#FFD700;text-align:justify; width:518px; float:left; margin-top:5px;}[/COLOR]
[COLOR=#333333]#navegacion {background-image:url(objetos/fondo.png); height:60px;}[/COLOR]
[COLOR=#333333]#navegacion li {float:left; list-style:none; margin:0px 20px 20px 0px;}[/COLOR]
[COLOR=#333333]#navegacion ul { margin-left: 85px }[/COLOR]
[COLOR=#333333]#contenido {background-image:url(objetos/fondo.png);width:1000px; float:left;}[/COLOR]
[COLOR=#333333]#contenido p {padding:5px 10px 5px 10px; text-indent:15px;}[/COLOR]
[COLOR=#333333]#contenido h2 {padding:5px 10px 5px 10px; text-indent:15px;}[/COLOR]
[COLOR=#333333]#contenido h3 {padding:5px 10px 5px 10px; text-indent:15px;}[/COLOR]
[COLOR=#333333]#texto {background-image:url(objetos/fondo.png);width:1000px; float:left;}[/COLOR]
[COLOR=#333333]#texto p {padding:5px 10px 5px 10px; text-indent:15px;}[/COLOR]
[COLOR=#333333]#texto h2 {padding:5px 10px 5px 10px; text-indent:15px;}[/COLOR]
[COLOR=#333333]#texto h4 {padding:5px 10px 5px 10px; text-indent:15px;text-align:justify}[/COLOR]
[COLOR=#333333]#productos {background-image:url(objetos/fondo.png) width:1000px; float:left;}[/COLOR]
[COLOR=#333333]#productos li {float:left; list-style:none; margin:0px 0px 0px 0px;}[/COLOR]
[COLOR=#333333]#productos ul { margin-left: 225px }[/COLOR]
[COLOR=#333333]#menu {background-color:#FFD700; width:150px; float:left; text-align:left; margin-right:30px;}[/COLOR]
[COLOR=#333333]#menu li {font-family:Comic Sans, cursive; font-size:1.2em;list-style:none; margin:4px 0px 4px 6px;}[/COLOR]
[COLOR=#333333]#galeria li { cursive;float:left; list-style:none; margin:0px 10px 10px 0px;}[/COLOR]
[COLOR=#333333]#galeria a {color:black;color:#00ff00;}[/COLOR]
[COLOR=#333333]#galeria a:active {color:black;color:#00ff00;}[/COLOR]
[COLOR=#333333]#referencia{font-family:Comic Sans, cursive;font-size:1.5 em;color:#00ff00;text-align:justify; width:800px; float:left; margin-top:10px;}[/COLOR]
[COLOR=#333333]#pie {background-color:brown;height:65px;}[/COLOR]
[COLOR=#333333]#pie li {list-style:none; font-size:12px;}[/COLOR]
[COLOR=#333333]#pieuno {background-color:black; float:left; width:500px;}[/COLOR]
[COLOR=#333333]#pietres {background-color:black; float:left; width:500px;}[/COLOR]
[COLOR=#333333]a {font-family:Comic Sans, cursive;font-size:1.2em; color:#00ff00; text-decoration:none ; display: block; }[/COLOR]
[COLOR=#333333]a:link {}[/COLOR]
[COLOR=#333333]a:visited {}{ text-decoration:underline; }[/COLOR]
[COLOR=#333333]a:hover [/COLOR]
[COLOR=#333333]a:active {}[/COLOR]
[COLOR=#333333]#pie a {font-family:Comic Sans, cursive;font-size:1.2em; color:#8B2500; text-decoration:none ; }[/COLOR]
[COLOR=#333333]#pie a:link {}[/COLOR]
[COLOR=#333333]#pie a:visited {}[/COLOR]
[COLOR=#333333]#pie a:hover {color:#FFF68F ; text-decoration:underline ; }[/COLOR]
[COLOR=#333333]#pie a:active {color:#FFF68F;}
[/COLOR]
/* Imágenes */
.img-enlace:hover {
cursor: pointer;
opacity: 0.5;
filter: alpha(opacity=50); /* Para que la opacidad funcione en Internet Explorer */
transition: 1.5s;
-webkit-transition: 1.5s;
-moz-transition: 1.5s;
}
Y tu menú de navegación déjalo así:
<div id="navegacion">
<ul>
<li><a href="..//index.html"><img class="img-enlace" alt="portada " src="objetos/portada.png" height="45" width="100" /></a></li>
<li><a href="./productos/productos.html"><img class="img-enlace" alt="productos " src="objetos/produtos.png" height="45" width="100" /></a></li>
<li><a href="./informacion/informacion.html"><img class="img-enlace" alt="informacion (49K)" src="objetos/informacion.png" height="45" width="100" /></a></li>
<li><a href="./presupuesto/presupuesto.html"><img class="img-enlace" alt="presupuesto (49K)" src="objetos/presupuesto.png" height="45" width="100" /></a></li>
<li><a href="./ofertas/ofertas.html"><img class="img-enlace" alt="ofertas " src="objetos/ofertas.png" height="45" width="100" /></a></li>
<li><a href="./carrosdemadera/carrosdemadera.html"><img class="img-enlace" alt="carros" src="objetos/complementos.png" height="45" width="100" /></a></li>
<li><a href="./galeria/galeria.html"><img class="img-enlace" alt="galeria" src="objetos/galeria.png" height="45" width="100" /></a></li>
</ul>
</div>