imshow background and/or colour to alpha


I am trying to do a plot where small images are used as markers at
definite coordinate positions (just to make myself clear: I am trying
to see how the shape of a polymer changes with different parameters,
so I want little "icons" of the polymer placed at the x,y of the

Doing that with imshow() , putting images as sub-matrixes etc. seems
doable. However problem is my images have a black background, and
correspondingly it seems that the RGB 0,0,0 value of the matrix is
shown -kinda correctly- as black (a zeros filled canvas is shown as

I wonder: Is there a default way to tell imshow() that I want a
certain colour (in this case, black) , and that colour only, to be
alpha? Or do I have to manually change the matrix?