In-app notifications are messages sent to a user while he/she is active in your app. When done right, they show a higher efficiency of engagement over other methods like Push Notifications, SMS or Email.
That said, creating visually pleasing in-app notifications with images is an increasing problem due to the varying resolutions and screen sizes of devices launched by Apple, Google, Samsung and other major mobile phone manufacturers.
This post will help you design the best possible in-app messages that can be used across multiple devices, irrespective of the screen resolution of the device.
Source: Growth Hackers