Honestly, designing a figure so that it looks right from the sky is pretty easy. What would be much harder would be to draw a figure so that it looks right from one and only one spot -- from the top of a nearby hill, for example. This would require a very sophisticated use of perspective, especially if the figure were carved on uneven ground. In that case, the easiest way to lay it out would be to use a strong projector to cast the image on the ground during a dark night and mark where the light fell.
Since suspicion always falls on aliens being behind all of man's early accomplishments, ask yourself -- if they wanted to leave a sign of their presence for future generations, wouldn't they have done something harder, something that would better show off their technical expertise?