A common usage pattern is to have the notify icon the same as the main window's icon. The icon is defined as a PNG file.
To do this, add the image to the project's resources and then use as follows:
var iconHandle = MyNamespace.Properties.Resources.MyImage.GetHicon();
this.notifyIcon.Icon = System.Drawing.Icon.FromHandle(iconHandle);
In the window XAML:
<Window x:Class="MyNamespace.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:local="clr-namespace:Seahorse"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Height="600"
Icon="images\MyImage.png">