First, it’s important to note that all Canvases are automatically set to the “Orientation Match” feature. This means that vertical images do not display in a horizontal orientation, and vice-versa. To change this, simply go to “Settings” (see “How do I access my Canvas’ settings?”)
If Orientation Match is off, and you have your Canvas set in a horizontal orientation, you can still view vertical images (and vice versa). However, there will be empty space around the image. (You can change the color of this ‘Letterbox’ under ‘Canvas’.)