save images with alpha and composition

hello.
I am trying to make image composition using captured image with alpha value from a part of webcam screen.
The image will be captured from blob detection and the alpha value will be given by time past from captured moment. the captured images will be stored in buffer with alpha value and coordinates, and all of images in buffer has to be composited on an one screen. in the end, the composition screen will be saved to an image file.
my questions are, how can I put the captured images with coordinates and alpha into the buffer, and display all images in buffer on the composition screen before save into the image file.

plus, can I add edge-blurred mask over the each images?