If you did add a second lock, it would be best in-line with the horizontal strake between the top and middle pane of glass. A third lock could go down under the handle, again in-line with the wood.
Adding more locks will do little for real security - There is no good spot for another lock. Additional locks on that door will be a visual deterrent at best, and a time-sink for you every time you use it.
You might get more mileage out of replacing the existing latchset with something modern, before adding a second lock.
If I wanted to enter via this locked door, I'd try:
- the handle first in case it is open, then a quick shove with a credit card at the lock's tongue, and a jiggle with a lock rake.
- break the glass nearest the lock and manipulate it from the inside - probably opens without a key from behind.
- sledgehammer the handle off and directly manipulate the square shaft inside with pliers.
One action is to replace the six single panes of beerglass with something more durable.
- Wood - will make the inside foyer darker during the day. Very hard to break. Glued, screwed/nailed and painted this will add rigidity to your door. Weight goes up too, making it feel more solid.
- Double-glazed inserts - still allows light, retains heat better, and is harder to break than single glass. Can be mirrored on the inside to add privacy.
You'd also want to replace that existing combo latch with a separate key-based deadbolt (it needs a key to open from the inside, and you take that with you when the house is empty)
Your existing door would not resist a brute force attack for long even if the lock was not targetted - a sledge hammer or sawzall would bust out the little horizontal pieces quickly, and if the middle vertical piece didn't fold up with the sledge, then the sawzall would take around 60 seconds to cut horizontally through it and then it folds down.
Ultimately your door is assembled from pieces - if you want it to be a lot stronger then replace the entire door panel with something solid, or at least with no holes in it.
Also consider uprating the door frame, and make sure the fastener screws you use are sufficiently long and that your new deadbolt goes home into the frame with minimal play.
Another option might be to add a second external door, like a screen door that uses strong mesh. This means opening two doors when you get home.
Afterwards go around your house and consider all the other entrances too. Garage doors tend to be weaker, where internal access gives a direct path into the house. Windows and sliding doors, cat/dog doors, celler/basements and attic/gable-ends can potentially be entry points.
Avoid overly-fortifying the Drawbridge and leaving your Postern gate unprotected.