HTML override

[FlutterYay]

I'm looking at this part of HTML/Source coding in my chrome element inspector. (Shown below) and I'm wondering if there is a way to override this div class so the image changes.
As a moderator we have access to source code at the bottom of the page, but we cannot modify any of the actual source code directly, we can only add new codes at the bottom. Is it possible to locate and overwrite this script through CSS or some sort of script?

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<div id="st-descr"><div class="banner"><img class="hide" original-src="http://fc00.deviantart.net/fs71/i/2...anner__fluttershy_by_dylandylan72-d4ks6yi.png" src="http://fc00.deviantart.net/fs71/i/2...anner__fluttershy_by_dylandylan72-d4ks6yi.png" style="display: inline; "></div>
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[FlutterYay]

I figured it out. I just needed to create a CSS file with this code.

/* Remove the existing banner */
#st-descr .banner .hide {visibility:hidden;}

/* Background image as new banner */
#st-descr .banner {background: transparent url

("<insert new image url>")}