No way to crop of corners with the Crop effect or any other effect.
But you can make the shape you want in the Titler and use it as a Track Matte.
The Frame Size is set to a horizontal Aspect Ratio. If your Clip (or Still Image) is larger than that, or the Aspect Ratio does not match that of the Sequence's Frame Size, the Clip will effectively be cropped to that Frame Size.
For instance, if you have a vertical/portrait Clip, and it is Scaled to fit, say the horizontal dimensions, the Video Frame will effectly crop off the top, and/or bottom. There would be no need to apply the Crop Effect, as pixels outside of the Frame will not be seen.
Now, if you Scale, to where the vertical dimensions match the Sequence's Frame Size, you will have black borders (the limit of the pixels in that vertical Clip) on the sides.
Ann has given you the route, if you want, say a rectangle with rounded corners, or perhaps an oval - Track Matte Keying with a Shape defining the image, like a rounded corner rectangle, or an oval.
I feel that I am missing exactly what you want to do, so maybe a screen-cap with annotations, showing what you wish to achieve, will lead to some exact steps to accomplish that.
Bill and Ann Thaks for the reply. You are right about scaling which would take a rotated image and have the slanted or crooked part outside the visible area. My problem is that I am putting the video clip inside a copy of a strip of film. Therefore the clip is scaled already to fit inside the film strip. When I need to rotate,the entire image is there. Perhaps, a matte as Ann suggested would be helpful in this case.
Now I understand what you are in need of, and also understand the "rounded corners." Depending on how you created the "film strip," you can probably use it, as a Track Matte, or take it into PS, to create the Track Matte from it, and then apply it to the PiP (Picture in Picture) Clips.
Good luck, and thank you for the clarification.