I'm not sure if you can do this in every browser but you can set the css property of the specified img.
Try to work with jQuery which allows you to make css changes much faster and efficiently.
in jQuery you will have the options of using .animate(),.fadeTo(),.fadeIn(),.hide("slow"),.show("slow")
for example.
I mean this CSS snippet should do the work for you:
img
{
opacity:0.4;
filter:alpha(opacity=40); /* For IE8 and earlier */
}
Also check out this website where everything further is explained:
http://www.w3schools.com/css/css_image_transparency.asp