I've just added the ability to remove the gradient background from the slideshow in theme settings (under Homepage).

For existing Vanity users, you can do this by going into styles.css.liquid and removing the border and background attributes for "#banner .images". It should look like this when you are done:

#banner .images {
    height: 300px;
    width: 100%;
    float: left;
}

You may also want to adjust the margin for each image in "#banner .images div" since it will be indented by 5px;