Hi John,
You are right, currently I cannot see any option to control the transparency of an image inserted directly in the Layout canvas. The workaround as you have probably already noted would be to add the image firstly in a new map window and then to bring it into a frame in the layout window.
It is sounding to me like it may be an enhancement request. Looking through our current ideas, I do not see any similar requests - can you please raise this on our Ideas Portal when you get a chance?
Dave