iOS App Icons
From Xojo Documentation
There are also a wide variety of app icon sizes for the various iOS devices. When you create a new iOS project, an empty App Icon item is added to your project. When you click on it, you'll see a list of available app icons sizes.
Drag the images you want to use to the appropriate item. Note that @2x and @3x icons are used on retina devices and are double or triple the resolution of @1x images.
You can use the iOS ImageMaker utility to create all the icon files for you from a single image or you can create them yourself (see below).
Your app icons should be opaque with no transparency.
If you have deleted the App Icon project item from your project you can add it back by selecting Insert App Icon from the Insert menu.
If you created an App Icons folder using earlier versions of Xojo, you should migrate it to an App Icon item by right-clicking on it an selecting "Convert" or by deleting the App Icons folder, selecting Insert ↠ App Icon from the menu and then dragging in your images to the appropriate sections.
For a description of other App Icon recommendations from Apple, refer to the iOS Human Interface Guidelines section on App Icons.
iOS ImageMaker Utility
The iOS ImageMaker utility included with Xojo can be used to quickly create all the file sizes you need for your App Icon and Launch Images. This utility is located in your Xojo folder in Extras/iOS Utilities
On the App Icons tab, choose the icon image to scale to the other sizes. For best results, select an icon that is 180x180 or larger. Then select the icons sizes you want and click the Create Icons buttons. The tool will create a folder containing the icon files. You can then drag each icon to the appropriate image size of the App Icon Image Set in your iOS project.