the easiest way to do this is with a CSS Insert. If you look a a page and then at the source, you will see that the comment icon is using the class 'comment-icon'. if you again look in the source for 'comment-icon, you will find this CSS
HTML Code:
a.comments-icon:link,
a.commentss-icon:active,
a.comments-icon:visited {
background: transparent url(http://www.derekbeck.com/1775/wp-content/themes/atahualpa353/images/comment-gray.png) no-repeat scroll center left;
}
a.comments-icon:hover {
background: transparent url(http://www.derekbeck.com/1775/wp-content/themes/atahualpa353/images/comment.png) no-repeat scroll center left;
}
(oh look...there's a bug...the active should be a.comments-icon:active not a.comments
s-icon:active - time for a BUGFIX)
so in your case you can duplicate this and add it to CSS Inserts and change the image to what ever you want.