It seems OpenCV does not provide very much support for transparent images. It has images with 4 channels, which are treated as BGRA images, with the fourth channel being the Alpha channel. It also supports saving these images using the PNG format, yet it does not support properly displaying these images in highgui, nor does it support combining these images.
I have looked a while for a way to combine a (BGR) background image with a (BGRA) foreground image, with semi-transparent images, but was unable to find one. So, I wrote my own and since I couldn't find a solution on the internet, I'll bet I can save some people some time by sharing my implementation, which is as follows: