TWebImageControl transparancy

I have stacked TWebImageControls and I want to control the transparency of the top one. I can do this with a TWebPaintBox with the following code on the canvas. I want to do something similar. I tried using the Canvas of the image control but the Context is null.

Canvas.Context.globalAlpha := FTransparency / 100;

A TWebImageControl has no canvas. It is a class for the HTML IMG element.
I'd suggest to use PNG or SVG images with transparency.