If you are content to expand the photo so it fills the width of your video, at the tradeoff of losing some off the top and bottom of the photo, you can also easily accomplish this by pan/crop.

Click on the Pan/Crop icon of your photo on the timeline. This opens up a window that shows your photo with a dotted-line frame around it and an "F" in the middle. This dotted line frame is a rectangle, in the same shape as your original photo.

Right-click anywhere inside the cropping frame and select "Match Output Aspect". The cropping frame changes to a rectangle the same shape as your video project, and zooms in a little so that the photo fills the frame. You can see the top and bottom of your photo being cut off.

While here, you can adjust the frame up or down if needed by left-clicking inside the box, and then using arrow buttons to move the box up or down. Maybe you want to keep more of the lower half of the photo, and cut out more of the sky, for example.

You can also zoom in more, pan across the photo, etc... The key here is simply that the shape of what you get will match the shape of your video, and should fill the frame of your video, as long as the photo fills the dotted-line cropping frame.

If you ever want to go back to the original shape of your photo, open up the Pan/Crop window, right-click in the box, and select "Match Source Aspect". This returns the cropping frame to a rectangle the same shape as your source photo.