While updating the screenshots of one of our apps via the new iTunes Connect interface I was getting the following error:
Images can’t contain alpha channels or transparencies.
I was able to strip out the alpha channels off the App Store assets using ImageMagick's mogrify command.
$ mogrify -alpha off -format png screenshots/*.png
mogrify command is an in-place batch processing utility. Extra care has to be taken because by default it will overwrite your images.