Using jQuery to Add a Class Attribute to images, etc. on Page?

Li'l Help? Apologies in advance due to my jQuery.noob status, but I'm drawing a blank and about out of ideas on why this isn't working. Maybe someone here can see something I'm not? I'd surely appreciate any help.

The Issue

This webpage has several images on it (also paragraphs, etc.) that I want to add a class to. They don't currently have a class on them.

Why I want to Add a Class

I want to use CSS3 to target that class and style anything that has it. More specifically this is going to be a pop-in menu and I'd like to adjust the opacity of it and animate its entry & exit.

How I'm attempting to do This

<script type="text/javascript" src="jquery-1.7.2.min.js"></script>
<script type="text/javascript">
    $("document").ready(function() {
        $("img").addClass("menumain");
                   alert("Here I am!"); //This alert will go when I figure this out, I just added it to see if it pops up (it does).
    });
</script>

**I've Also Tried it This Way (Note: All of my images contain the string "menumain-" in the file names)**

<script type="text/javascript" src="../jquery-1.7.2.min.js"></script>
<script type="text/javascript">
    $("document").ready(function() {
        $("img[src*=menumain-]").attr({class:"menumain"});
        
    });
</script>

So What Happens When I Try it?

Nothing. I load the page, (that alert does pop up, which I dismiss) then expand out the body using firebug and / or Safari developer tools, etc. and drill down to the images on the page. I see the image attributes, but I don't see where the class was added, much less defined as "menumain&quo

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