It’s easy to change the “SALE” text in the bubble in Sellegance theme with your own image.

Paste the following in catalog/view/theme/sellegance/stylesheet/custom.css

.onsale {
background: transparent url('../images/new_on_sale.png') no-repeat;
-webkit-box-shadow: none;
-moz-box-shadow: none;
box-shadow: none;
padding:0;
/* change height/width according to your image */
height: 35px;
width: 35px;
/* hide the SALE text. */
text-indent: -9999px;
}

If you only want to add a background image and keep the text remove text-indent: -9999px;

The full path of the example image is catalog/view/theme/sellegance/images/new_on_sale.png. Of course, you need to change the height and width values to match your image dimensions.