The image's alpha channel would be multiplied by the alpha value. So, an image with alphas ranging from 0.2 to 0.8 combined with alpha=0.5 would effectively be treated like an image with alphas in the 0.1 to 0.4 range. This seems reasonable, and it's already the way the backends that don't require this compositing behave.

But now that you point it out, this test case should probably involve that. I'll update it.