Convert GE Load Center to main breaker
Holy smoke, this is a "Rule of Six" panel!  

A Rule of Six panel has a "Main Breaker" area with up to six main breakers. All must be shut off to shut off the power.  All six have their buses energized at all times.  Then, one of those breakers feeds an "internal subpanel" area with any number of breakers. 

In this case, your panel is built as a "Rule of Three".   The first two breakers are #1 in the upper left, #2 an essentially identical breaker in the 2 full spaces in the upper right, and #3 the mid-left breaker marked "MAIN".   

Note that the #3 breaker actually sits in the subpanel area and is fed via those two fat wires.  It is backfeeding the subpanel area.   Those two wires are hot at all times and cannot be shut off without pulling the meter. 

You are saying "But wait. The #2 breaker is not as you described.  It is narrow "double-stuff"  breakers not a full-sized one."  Yes, and I don't want to know what kind of black magic got you there.  It looks like someone broke off some of the physical gating to separate the Rule of Six area from the subpanel area.  It is bad. 

Look at the dryer circuit.  It has one leg in the Rule of Six area, and one leg in the subpanel area.  If you shut off the "MAIN" breaker, half the dryer would be hot and half dead.  Very bad!  

So it's worse than "need to squeeze in another breaker", it's actualy you need to move the dryer breaker away  so you can restore the breaker in that location to a traditional full-size breaker as is required by the diagram.   

Honestly at this point, you might as well go "in for a penny, in for a pound".  Install a subpanel right next to this panel, relocate the dryer feed to it, relocate the garage subpanel feed to it, and add any new circuits there.

Feel free to use a GE Qline panel that supports double-stuffs, so you can keep using the same breakers.  However you will probably not be able to use double-stuff breakers going forward because AFCI or GFCI is required on almost every circuit. So get a realy, really, really big subpanel!  Spaces are cheap.  

Bonus points if you plan it so you can cut over the subpanel to be a future main panel.  

Adding a main breaker here is simply not an option (it already has 3) and this panel is obsolete (being Rule of Six, nothing wrong with GE Qline). 

By the way, whoever the genius is who stuck numbers on the breakers, did not read the panel labeling.  It indicates the numbering.  Note how there is only 1 number for the upper  right breaker, that is because the listing and labeling does not permit multiple breakers there. 

Also whoever put the double-stuff in the upper right, don't invite him back either. he clearly mutilated the panel in order to make that dryer breaker go there.