Div Class – location

Ok so this is a bit of a noob question.

Im self taught and not hugely technical so need some help.
I’m basically trying to update a friends website and along with a number of changes ive managed to make, they want a couple images on the homepage changing…. sounds simple enough…. but i cant find where it is pulling through the image from. and i’ve looked everywhere i could think of.

on the page wher the image is i have this:

<div id=”top”>
<div id=”top-left”>
<div class=”studio promo1″>
<h2>Studio Hire</h2>
<p>random text ??</p>
</div>

and its the <div class=”studio promo1″> is the image i need to change but i cant find where this div class studio promo1 is being pulled from. ive checked posts, pages and everywhere else i can see.

Anyone got any ideas where this is pulling from or where i could look?

thanks