Apple has released new version of iTunes Connect & I got an error message when I tried to set Screenshots on itunes connect for my app.
"Images can't contain alpha channels or transparencies."
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~ Asked on 2014-09-05 08:43:52
AFAIK png with transparency is not allowed. use jpg OR update your png (photoshop or whatever you using to create the png) and delete the transparency areas. if you work with shadows, use jpg, that will do no headaches.
~ Answered on 2014-09-05 09:06:00
I've found you can also just re-export the png's in Preview, but uncheck the Alpha checkbox when saving.
~ Answered on 2014-09-08 15:06:46