While inserting the image in the XAML, we often use the relative URI (Images/image1.png, in our case) as it is relative to the application. You can also assign it more verbosely as pack://application:,,,/Images/image1.png, which is generally used while assigning the image source from the code behind.
You can also use the Visual Studio editor to assign the image source. To do so, right-click on the image from the XAML designer view and go to its Properties. From the Properties panel, click the dropdown arrow, as shown in the following screenshot, to select the desired image from the available items in the list:
