Icons in Apps
App icons are convention based, so there is no configuration for it.
To give your app an icon, place a file called
app-icon.png in the root folder of your app.
It should be square, and at least 200x200 Pixels, but we recommend it's at least 500x500 pixels.
Icons for Views / Templates
This is also convention based, there is no configuration for it.
To give your views/templates an icon, add an icon file with the same name as your template file.
So if your template is called
_overview.cshtml your icon should be
Icons for Content-Types
Content-Types don't have a folder dedicated to them, so this is configuration based. Just edit the Content-Type Metadata and on the Icon-field, drop the image you want for your content-type.