You can adjust an image's alpha in script by setting the baseColor attribute of the Image's material. For example:
script.myImage.mainPass.baseColor = new vec4(1, 1, 1, 0.5);
This code sets the image's alpha to 0.5. You can read more about this technique in the Image guide.
Tip: you can figure out the scripting property of a material by hovering over its field name in the Inspector panel.