change image opacity using javascript

In fact, you need to use CSS.

document.getElementById("myDivId").setAttribute("style","opacity:0.5; -moz-opacity:0.5; filter:alpha(opacity=50)");

It works on FireFox, Chrome and IE.

You can use CSS to set the opacity, and than use javascript to apply the styles to a certain element in the DOM.

.opClass {
    opacity:0.4;
    filter:alpha(opacity=40); /* For IE8 and earlier */
}

Than use (for example) jQuery to change the style:

$('#element_id').addClass('opClass');

Or with plain javascript, like this:

document.getElementById("element_id").className = "opClass";

Supposing you're using plain JS (see other answers for jQuery), to change an element's opacity, write:

var element = document.getElementById('id');
element.style.opacity = "0.9";
element.style.filter  = 'alpha(opacity=90)'; // IE fallback

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