If you don't already have Firefox web browser installed, get it. Second, install the firebug plugin.
You find some video tutorials on YouTube on how to use it. With Firebug, you'll be able to inspect your web pages, and quickly find the answer to questions like this. You'll be able to inspect all css for objects on your page, and quickly find out where the file is on your server. It is important that you disable all caching for your site when working with Firebug. Learning how to use Firebug, will make your life a lot easier. With firebug, you can inspect that button, and find the path to the image in about 5 seconds. That's a whole lot faster than asking the question in these forums, and waiting for someone to answer.
Sometimes, you won't get a direct answer,. Anyway, find where the image is stored on your server, and upload a different one in its place, overwriting the original.
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