Slight crack on my Invisalign upper retainer. Should I keep wearing it or get a new one? (Photo)

Wearing the retainer with a small crack should not cause any major issues until it cracks enough that it can cut up you gums or not hold the teeth in place effectively.  Most likely you are alright to continue to wear the retainer for now but I would definitely look into having a replacement made so that when it does break further you are not waiting to get a new retainer made.

Probably still fine but you should make plans to get a replacement in the next few months. It is easy to duplicate the retainer with just a phone call to the dentist who treated you.

Whenever you get a crack in a clear retainer, it is time to replace it.  It loses a lot of the force needed to keep your teeth straight.  As the crack gets bigger it becomes more of an issue.  Pinpoint holes tend to not be a big problem.  You'll get these in the canine area.