Yes, it is possible to populate the XRPictureBox in a web application at runtime.
If I understand you correctly, you wish to click a button (or do something similar) and change the image at this moment.
In this case, I suggest that you set the XRPictureBox.ImageUrl property.
Please note,that the report is not cached on the server, so this property will be reseted to its default value once this report is requested from the server by the client application.
To work around this problem, I suggest that you save this property value to the Session object and restore it when needed.
The sample project attached shows this approach in action.
I hope, it will be helpful to you.
Please feel free to contact us in case of any difficulties. We will be happy to assist you.