(This feature is in beta. Please send questions/comments to firstname.lastname@example.org)
VoltBuilder can generate icons and splash screens in all the required sizes for your app. Just supply an icon and a splash screen template, and VoltBuilder will take care of the rest. The generated images will be added to your project and your config.xml file updated. Android, iOS and Windows are supported.
Not only does this make the production of your app easier, it also reduces the upload size to VoltBuilder. When new image sizes are required by platforms, VoltBuilder will automatically add them.
Of course, if you are happy with your current images, you can keep using them.