I wrote an ArcGIS Pro addin with the SDK to show notifications.
Since Version 2.2 showing costum images in notifications should be possible. How can I reach them, if they are put to the image folder an should be shown in the internal class Module1.
notification <SPAN class="operator token">=</SPAN> <SPAN class="keyword token">new</SPAN> <SPAN class="token class-name">NotificationItem</SPAN><SPAN class="punctuation token">(</SPAN>dateNews<SPAN class="punctuation token">,</SPAN> <SPAN class="keyword token">false</SPAN><SPAN class="punctuation token">,</SPAN> thema<SPAN class="punctuation token">,</SPAN> NotificationType<SPAN class="punctuation token">.</SPAN>Custom<SPAN class="punctuation token">,</SPAN> date<SPAN class="punctuation token">:</SPAN> myDate<SPAN class="punctuation token">,</SPAN>
customImage<SPAN class="punctuation token">:</SPAN> <SPAN class="string token">"pack://application:,,,/Module1;component/Images/globe30.png"</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">;</SPAN>
NotificationManager<SPAN class="punctuation token">.</SPAN><SPAN class="token function">AddNotification</SPAN><SPAN class="punctuation token">(</SPAN>notification<SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">;</SPAN><SPAN class="line-numbers-rows"><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN></SPAN>

This toastimage is shown with this code:
notification <SPAN class="operator token">=</SPAN> <SPAN class="keyword token">new</SPAN> <SPAN class="token class-name">NotificationItem</SPAN><SPAN class="punctuation token">(</SPAN>dateNews<SPAN class="punctuation token">,</SPAN> <SPAN class="keyword token">false</SPAN><SPAN class="punctuation token">,</SPAN> thema<SPAN class="punctuation token">,</SPAN> NotificationType<SPAN class="punctuation token">.</SPAN>Custom<SPAN class="punctuation token">,</SPAN> date<SPAN class="punctuation token">:</SPAN> myDate<SPAN class="punctuation token">,</SPAN>
customImage<SPAN class="punctuation token">:</SPAN> <SPAN class="string token">"pack://application:,,,/ArcGIS.Desktop.Resources;component/Images/ToastLicensing16.png"</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">;</SPAN>
NotificationManager<SPAN class="punctuation token">.</SPAN><SPAN class="token function">AddNotification</SPAN><SPAN class="punctuation token">(</SPAN>notification<SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">;</SPAN><SPAN class="line-numbers-rows"><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN></SPAN>
Thanks Karsten