" Canvas.Backdrop=SomePicture" does not work inside a loop

Why if I have " Canvas.Backdrop=SomePicture" inside a loop the picture does not appear?

Can you share a sample project to see your loop?
That way it will be easier to understand your need and what you want to do.
You can use dropbox, onedrive, wetransfer or similar to share your project.

Generally, the user interface does not update until the current event ends.

Instead of using a loop, try using a repeating Timer, where the image is updated each time the Timer.Action event is triggered.