Not sure what you are actually asking.. you want a plugin or css declaration that will create rounded corners on your images?
I'm fairly sure such a thing does not exist, and corners must be rounded with a graphics editing app. I guess you could overlay a div with background that has gif corners (that is, the parts 'outside' the rounded corners would be filled in with the same as your background colour), but probably need one of these gifs for every image size that you will be putting rounded corners on...
lol, is easier and avoids potential bugs if u just use a graphics app... if you have photoshop, or something similar, use the rounded rectangle and create a layer mask with it.
good luck and have fun with that,