Since Adobe Revel just replicated the thing Google+ did, and they're both wrong, I thought I would say this:
If you have two images over a black background and you want to crossfade between them, please DO NOT fade out the first one as you fade in the second.
(Exception: you can fade the first one out, quickly, at the very end of your animation, if the aspect ratios are different.)
The reason is this: in the middle of your animation, if you do it the Google+/Adobe way, you will have:
- image A at 50%
- image B at 50%
Total opacity: 75%
And you didn't really mean to have 25% of the background showing through there, did you?
Crossfading between two images this way will leave you with a 25% darker image in the middle of your very fast crossfade, and that's very distracting and looks bad.
You don't want intensity to change in a crossfade, unless it's very slow and fades to black on purpose.