When one first creates a new project, that dialog lets you point to some external .PNG file, and then when that dialog completes, it generates 4 different pixel-sizes of ima
Click "File > New > Image Asset"
File > New > Image Asset
Asset Type -> Choose -> Image
Browse your image
Set the other properties
Press Next
Next
You will see the 4 different pixel-sizes of your images for use as a launcher-icon
Press Finish !
Finish