.watermark-img { position: relative }
.watermark-img:after { content: " "; width:20px; height: 20px; position: absolute; right: 0; bottom: 0 }

<div class="block"><img class="watermark" src="image.jpg" alt="" /></div>

.block {
  background: url(watermark.jpg) right bottom no-repeat;
  width: 100px;
  height: 100px;
}