<span class="dissolve img1"><img src="http://www.pmob.co.uk/temp/images/zimg6.jpg" alt="Sunset" width="300" height="300"></span>

.dissolve {
  width: 300px;
  height: 300px;
  margin: 25px auto;
  display: inline-block;
  transition: all 0.6s;
  background: url(http://www.pmob.co.uk/temp/images/zimg5.jpg) no-repeat 0 0;
}
.dissolve img {
  opacity: 1;
  transition: opacity 0.6s ease-in-out;
}
.dissolve:hover img {
  opacity: 0;
}