Apple Touch Icon Guidelines. Can be a relative path. Restrictions and other terms apply.
Since 22×22 is the smallest size the icon will be. The os will scale the most appropriate icon to the correct size, so you don't need to create any specifically sized icons for android devices. Ios will display those screens when your pwa is being opened as well as when it's in the background.
When ios safari users add progressive web apps (pwas) to their home screens, the icon that appears is called the apple touch icon.
Android tablets [via chrome] prefer a 192x192 png icon. This issue can be mitigated by declaring the icons in the html code (this is necessary for android anyway), but following apple conventions is probably the best move. Once you have created your icon, you can then upload it into the content >> files area within your create account (not the images area). For example, someone once told me he wanted to use the nearest neighbor algorithm to scale down his icon (it was pixel art;