Firing from Cover With an Imaging Device (p. 177, emphasis mine) is stuff like pointing your gun around a corner. You can only do it with an imaging device and it assumes you're firing from total cover. If you don't have an imaging device, you'd use the Blind Fire penalty.
Take Cover is an action that can result in either Good Cover or Partial Cover, depending on what the GM decides is available. Since Partial Cover assumes only 25-50% of the defender's body is covered (so standing behind a waist-high wall, for example), I wouldn't require a player to spend another action on reestablishing cover in most cases--the wall isn't going anywhere. Good cover is 50%+ and very well might require additional Simple Actions to reestablish, depending on the circumstance. A human taking cover in a fairly deep doorway from corp sec down the hall might qualify for this bonus, but may also need to spend a Simple Action to duck back into the doorway after firing. Obviously, if you're getting this bonus because you're prone, you don't need to reestablish it.
Weird rules note that I mentioned in the errata thread: If you take Partial Cover behind soft cover (so no barrier rating to speak of), you are actually easier to hit than if you took no cover at all. For some strange reason, cover negates the "ties go to the defender" rule, which means attackers need one fewer net success to hit you.