Hello,
Why not add some picture in your thumbnail div via Css command with if needed specific selector (for example if you want restricted image display to one or several category).
With this and some "positioning" command to keep the new image in the top-right corner for example, something like this can do the trick :
.category_class .thumbnail_div_class a:before {
content: url("http://YourWebsiteUrl/images/color/ImageName.png");
position: absolute; / to better move your new picture */
left: 85px; /* To deplace to the right the pic */
top: 5px; /* To deplace to the bottom the pic */
}
You can learn how to customize your frontend Css,
here
Hope this will fit your needs.
Regards