For a proper visual design of an application, it's better to follow rules below:
- Orientation: Which orientations are supported by mobile device. Define the rules of orientation changes.
- Resolution: The resolution should be at least 1080×1920 (1920×1080) or vector design.
- File Format: It is easy to work with an advanced designer program such as Adobe Photoshop. The app developers may easily crop the images from PSD files. The cropped images are better to be saved in PNG format for each different level of density of work.
- UI Objects: It would be hard to crop the images if they are in same image. Layered image should be created and layers should be easily separate the UI objects
- Design Guide: How the UI objects, images will behave when the screen size and aspect ratio is different than designed. General and specific rules has to be defined.
- Element States: The UI elements can be in different states when they are enabled/disabled or normal/pressed/focused. General and specific rules has to be defined for how the background changes, how text color changes...
- Font: The working font sizes and font types has to be defined. The used font should support the middle ranges of font sizes in pixel. For example, the used fonts should support font sizes from 5 pixel to 50 pixel with 1 pixel iteration, that support has to be observed by eye.