Can't upload screenshots to iTunes Connect

Question

I want to upload an update for my app. The build is already approved and I want to upload screenshots for all the devices. Now the problem is that when I want to upload the screenshots iTunes Connect it says

Screenshots may not contain transparencies or alpha channels.

Same goes for my large icon I wanted to add to a new app I created. There were DEFINITELY no alpha channels or transparencies in any image I tried to upload.

So does anyone know how to fix this?

(I thought iTunes Connect had an error so I waited a day but that did not help...)

Answer 1

Convert the images to JPG, generally PNG images have an alpha channel set.

Alternatively, you can re-export the images using Preview on Mac and uncheck the "Alpha" checkbox.