You can use border-radiusonly on the two left corners and combine two values each, as in this example (you have to try around a bit to find a good combination):
.outer {
width: 500px;
background-color: #ffffd;
overflow: auto;
}
.outer img {
float: right;
border-top-left-radius: 30px 50%;
border-bottom-left-radius: 30px 50%;
}