I like the convenience of being able to crop jpg/png images with Preview, but I would also like to be able occasionally to restrict the aspect ratio of the cropped image. This is useful, for instance, if I want to prepare images for printing in 4x6 or 5x7, while avoiding any surprise cropping made by the photo lab.

I am hoping there is a quick and easy way to do this (i.e., without needing to invoke the behemoth of image editing for this elementary task). Are you aware of one?