Changing an image's source in Windows phone

I'm developing a metronome application, I made a bunch of images for it to display the rhythm number, but I don't know how tho load the images in the first place if I even need. I tried with this:

tactImgList[0] = new BitmapImage(new Uri("Assets/Images/1.png"));

And so on the rest, but I'm always getting a URI format exception.

What am I doing wrong? I tried to google my problem, but nothing.