1. Congratulations, dummy. You have just let criminals know where they can steal a quick and easy gun anytime you park somewhere and have to leave the gun in the car because the premises are off-limits to guns. Generally, the chances of breaking into a car and finding a gun are random and unpredictable; but, you just removed all doubt.
2. Congratulations, moron. You have just created a bad mental image of all persons with a CCW/CHP/CCP/CPL/etc. to anyone who looks at that photo. Heck, you even matched some people's stereotype of us permit holders (angry person who is just waiting to use his/her gun in a road rage incident).
3. Congratulations, idiot. You have demonstrated that you may eventually use your gun just becuase someone made you angry. Now, if you get involved in a borderline self-defense situation and the prosecutor finds this photo, you just lost any benefit-of-the-doubt or credibility you may have had.
In short: if the person who wrote that on their car is reading this, amend your ways now.