Before you add your photo to your project, go to Edit/Preferences and, on the General page, uncheck Scale Still to Frame Size. Then add your photo to your project and place it on the timeline.
This will display your photo at its full resolution rather than artificially shrinking it to fit into your video frame. Your photo, however, might be a lot larger than your video frame, depending on the photo's size. A 4000x3000 pixel photo will be quite a bit larger than your 1920x1080 video frame. You can then resize it in Applied Effects/Motion.
There is no other way to fill your frame horizontally -- and that's true on any video editing program in the world. Your photo is 4:3 and your video frame is 16:9. You can either have it set to fit in the frame and not fill the frame completely or you can manually set the size of the photo yourself.