making a straight image rounded

This may be a bit hard to explain but here goes:

I've got an image that is basically banner style:

--------------------------------
|Flag1|Flag2|Flag3|Flag4|
--------------------------------

It has four different country flags on it. What I'd like to do is turn that image into a circular image. Hopefully you know what I mean. I've tried to use the shear filter but can't get it to look right. Any ideas? If I haven't made myself clear, let me know and I'll try harder.

Thanks,

Ryan

..you might want to try the filter "polar coordinates"