Couple of nice open source tools from Felix Krause specifically for working with app screenshots.
The first tool is an excellent complement to Deliver called Snapshot.
Snapshot uses UI automation and allows you to automatically take screenshots within your app, automatically removes the status bar and waits until the appropriate moments so there’s no unwanted images such as those of loading indicators. This makes it easy to create screenshots for multiple languages, and you only need to create the config one time. Snapshot can also integrate with deliver and upload the screenshots automatically.
You will need to create a UI Automation script to go through your apps interface.
You can find Snapshot on Github here.
The second tool is FrameIt which allows you to add a frame to your iOS screenshots automatically in batches for iPhone 6 Plus, iPhone 6, iPhone 5s, and iPad air screenshots.
You can find FrameIt on Github here.
As stated in the FrameIt readme you can find the iPhone and iPad device art over on Apple’s marketing guidelines page.