Alpha Key

I Have a charting component that Generates Charts (obviously)

GIFs to be more specific.

But it is Quite Limited Graphicly so I would like to append an image in the Background of the Chart . so What I have is the following : 2 gifs (1 chart,1 background)

I want to replace the white (default) background of the chart by the image I have.

So if there isnt a straight forward method to do this ( like multiplexing 2 images ) maybe there is a way to set through an API code the White to be transparent and I can handle it from there.

Thanks.