Square:
{border-radius:0;}

Circle:
{border-radius:50%;}

Diamond:
parent{width:[hypotenuse of 1/2 image size];height:[hypotenuse of 1/2 image size];margin:auto;transform:rotate(45deg);overflow:hidden;}
child{transform:rotate(-45deg);transform-origin:[varries];}

Octagon:
parent{transform:rotate(45deg);overflow:hidden;}
child{transform:rotate(-45deg);}

Asymetrical:
{border-radius:50% 50% 0 50%;}

Blob:
{border-radius:6.25% 12.5% 25% 50%;}