Add png requirements to icon guide (tauri-apps#1113)

* Add png requirements to icon guide

* Update docs/guides/features/icons.md

Author: FabianLars

docs/guides/features/icons.md

@@ -31,7 +31,7 @@ USAGE:
     cargo tauri icon [OPTIONS] [INPUT]
 
 ARGS:
-    <INPUT>    Path to the source icon (png, 1240x1240px with transparency) [default: ./app-icon.png]
+    <INPUT>    Path to the source icon (png, 1024x1024px with transparency) [default: ./app-icon.png]
 
 OPTIONS:
     -h, --help               Print help information
@@ -60,8 +60,11 @@ By default, the icons will be placed in your `src-tauri/icons` folder where they
 
 ## Creating the icons manually
 
-If you prefer to build these icons yourself (if you want to have a simpler design for small sizes or because you don't want to depend on the CLI's internal image resizing), the required layer sizes and names for the [`icns`] file are described [in the Tauri repo] and the [`ico`] file must include layers for 16, 24, 32, 48, 64 and 256 pixels.
-For an optimal display of the ICO image _in development_, the 32px layer should be the first layer.
+If you prefer to build these icons yourself, for example if you want to have a simpler design for small sizes or because you don't want to depend on the CLI's internal image resizing, you must make sure your icons meet some requirements:
+
+- `icon.icns`: The required layer sizes and names for the [`icns`] file are described [in the Tauri repo]
+- `icon.ico`: The [`ico`] file must include layers for 16, 24, 32, 48, 64 and 256 pixels. For an optimal display of the ICO image _in development_, the 32px layer should be the first layer.
+- `png`: The requirements for the png icons are: width == height, RGBA (RGB + Transparency), and 32bit per pixel (8bit per channel). Commonly expected sizes are 32, 128, 256, and 512 pixels. We recommend to at least match the output of `tauri icon`: `32x32.png`, `128x128.png`, `[email protected]`, and `icon.png`.
 
 [`tauricon`]: https://github.com/tauri-apps/tauricon
 [in the tauri repo]: https://github.com/tauri-apps/tauri/blob/dev/tooling/cli/src/helpers/icns.json