Great!
You can read next wiki article about using aerial imagery:
https://wiki.openstreetmap.org/wiki/Using_aerial_imagery

GPX - is the file format for recorded GPS location.

But the idea is simple: 
1. enable GPS tracks layer (in iD editor it's under Background > Overlays > OpenStreetMap GPS traces)
2. now you'll see lines of recorder gps tracks uploaded by users. If there are many lines - they can be assumed as pretty accurate. 
3. Now you can shift background to align roads with GPS tracks
(in Id it's called "Imagery Offset" and located at the bottom of the same Background page).
4. at the moment images are aligned to GPS tracks - you can safely adjust actual objects to their images.

This must be done before any mapping using satellite images.

if there are no GPS tracks in your area, try to scroll around, search for major roads. Preferably find a place where tracks go in all directions so you can align both horizontal and vertical.
