If an image needs to stick to the bottom of the page, you'd usually just use position: fixed. But what if it needed to stick to the bottom of the page only while inside its parent & while the parent's Y offset was inside the viewport. In other words, if the user scrolls past its parent, the image element needs to stick to the bottom of the parent (i.e. position: absolute, bottom: 0), and if the user scrolls to the top of the page, the image element needs to stay on top of its parent (i.e. position: static)

<script type="text/javascript" src="http://code.jquery.com/jquery-1.7.min.js"></script>
<script src="/fixedElemScroll.jquery.js" type="text/javascript"></script>

$("img.need_to_scroll_inside_its_container").fixedElemScroll()

// if you want to specify the parent
$parent:  $this.parent()
// if the parent's element position is static, bottom position will be the difference between the bottom of the parent element
// and the bottom of the closest parent element with a position not equal to "static"
bottom: 0

Tested on IE7+, FF 3.6+, Chrome, Safari