This should be as simple as applying border-radius to one side of your image, playing with the values and image size to get the effect you require:
img {border-radius: 250px 0px 0px 250px; }
<img src="http://placehold.it/300x500">
More complex curves
Additionally you can get involved with more complex curves like this:
img {
border-top-left-radius: 100px 200px;
border-bottom-left-radius: 100px 200px;
}
<img src="http://placehold.it/100x300">