That image (along with the shadow) is part of the master sprite image (which you can see here). You need to replace the entire image, using custom CSS like this:
#top_search button {
background: url("link to image you want to use") no-repeat scroll top transparent; */
}
You can download the master sprite, extract the part you want using Photoshop or GIMP, and upload it to your media library. You can use the link you get there in the above CSS.