Have you considered simply using CSS and CSS media queries to specify the content of the image and then change the image's content once the screen size has dropped below a threshold specified in the CSS media query.
If you look in bootstrap.css, you will find the framework already implements many CSS media queries.
I put together a quick test, which ran perfectly in Chrome and IE.
<!DOCTYPE html>
<html>
<head>
<style>
#my-image { content:url("[insert path to your full size image]"); }
@media screen and (max-width: 640px) {
#my-image { content:url("[insert path to your small size image]"); } /* Once the screen drops below 640px, the new image will show */
img { border: solid 4px #000; } /* I'm apply some an additional border to help illustrate when the screen width is below 640px */
}
</style>
</head>
<body>
<img id="my-image" />
</body>
</html>
Important Note
You will need to modify the 'content:url' in order for the code snippet above to run.
Here's another Stack Overflow post that discussed this approach: Switching CSS classes based on screen size
I hope this helps.