You can change icon on label with your own image. To make it happen change the code below replacing image.png with URL of your image. Then simply add this CSS on plugin settings page.

.nksub-tab-icon {
background: url(image.png) center center red !important;
width: 50px !important;
height: 50px !important;
}

.nksub-tab-icon i {
display: none !important;
}