From Xojo Documentation
For iOS App icons, refer to UserGuide:iOS App Icons.
The Icon Editor is used to add application icons to desktop and web apps. Desktop apps display the icon in the file viewer for the OS (Finder, Explorer, etc.) and in other places (such as the Dock on Mac or the Task Bar on Windows). Web applications display the application icon on the app's loading screen.
To use the Icon Editor, select the App object in the Navigator and then click the icon next to the Icon property in the Inspector. The Icon Editor displays as a window with blank areas for icons of various sizes. You can drag icons files to these areas to set the icon for that size. You can also copy and paste icons between the sizes; they are scaled automatically for you. If you copy and paste an icon, be sure to also copy and paste its mask. To remove a specific icon size, click on its area and press the delete key.
You can also use the Delete, Paste, Import contextual menu when you right-click on any of the individual icon image areas to add or remove icons. Common image formats are supported, such as JPG, BMP, PNG, etc. You can also drag an ICNS or ICO file onto the window background (instead of an image area) to load and populate all the image sizes at one time. If you add an icon that has both the image and the mask (such as with most PNG files) then both the image and mask are added.
Use the Images/Masks/Preview selector to display the various components of the icon images.
For best results, be sure to add icons (and masks) for every available size. Different platforms use different sizes. If the masks are missing, the icons may not appear on some platforms.
On HiDPI screens, the next higher sized icon size is used. So if a 512 point app icon is needed on a HiDPI screen, then the 1024 pixel icon will be used.