Images blending/mixture/ph​otomontage

This is a small function designed to enter a small image, onto a bigger one. It can be handy in several cases- both for images and especially for videos (perhaps I'll write a wrapper function for video later on). It can be used for hiding an object (like faces of persons interested in anonymity). Another option is water marking.
Annotation, and image/video editing is also an option- adding an object to an existing image. The function receives a main- tile image, and a smaller image subject to insertion. The user specifies the inserted image position, the part of the image to be inserted, the way coordinates should be treated, and the opacity value. Image mixture is generated according to the above parameters, and the new image is returned.