We ran across this example of hidden gun storage at Covert Canvas. The image of a US flag lifts up with an adjustable support hinge to reveal space for your AR15, or other similar weapon, and additional mags and accessories.
Each of these Covert Canvas products comes with a magnetic lock that you can mount wherever you feel would work best for you. These locks are attached with adhesive, so aren’t meant for absolute security, but they will act as a strong deterrent for anyone who doesn’t know for certain there’s a gun behind.
Check out their site for a wide variety of other painting designs and smaller sizes for handguns.