Sure. To do that, you'll want to target specific widgets. So the first thing you should do is view the page source and find the ID value for the widget you'd like to update. Currently, your blog uses IDs of "categories-2" and "archives-4" for the categories and archives widgets respectively. Then you can use this CSS as a starting point to replace the widget titles with images:
#categories-2 .widget-title {
text-align: left;
text-indent: -9999px;
width: 36px;
height: 36px;
background: url('http://s.wordpress.org/about/images/logos/wordpress-logo-32-blue.png') no-repeat;
}
#archives-4 .widget-title {
text-align: left;
text-indent: -9999px;
width: 36px;
height: 36px;
background: url('http://s.wordpress.org/about/images/logos/wordpress-logo-32-blue.png') no-repeat;
}
I used a logo image in this example, and you can replace the url() values and update the width and height settings to switch to a different image.