Even if they bothered with the expense of placing satellites in orbit to photograph LV-426 through its pea soup atmosphere, the derelict could have easily been mistaken for a natural rock formation.
But how did they not notice the Derelict? It is a good question honestly, but the answer's that the film did not see technology getting to a point where you can easily photograph an entire planet, never mind it happening in the modern day.
My guess is that it was surveyed later and selected for colonization simply because it could be terraformed into a habitable planet. The company co-financed the whole thing against mineral rights.
That doesn't mean they scanned every square inch of the surface first. The derelict was probably next to impossible to detect due to thick cloud-cover and the mountainous terrain.
Was it all a scheme? How did they not notice the derelict on such a small planet? How did they find out about the planet when the only ship that knew about it was the Nostromo?
These are probably old questions but I can't seem to remember the film answering them.