Why is the screen black when I try to take a photo?
On Apple devices, you need to grant the app access to the camera, in the Settings app > Privacy.
How can I take better quality photos than with the built-in camera?
Use a good quality digital camera. If you have an iPad, you can buy an Apple iPad Camera Connection Kit from the Apple Store, and connect the camera's USB cable directly to the iPad: you can then import any photos from the camera directly into the Photos library on the iPad.
If you don't have a connection kit, or for an iPhone or iPod touch, you can transfer photos from the camera to your Mac or PC, and then synchronise to your mobile device using iTunes.
Alternatively, email photos as attachments to yourself: when you open the email on your mobile device, you can save the attachments to the Photo library (on iOS) or Gallery (on Android).