Code:
<img src="klematis.jpg" 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" />

<img src="klematis2.jpg" 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" />
The onmouseover attribute defines what will happen when the mouse pointer moves over the image. In this case we want the image to NOT be transparent when we move the mouse pointer over it. The syntax for this in Firefox is: this.style.opacity=1 and the syntax in IE is: this.filters.alpha.opacity=100.