add img to security light post

posts/installing-a-solar-powered-security-light.md

@@ -66,6 +66,8 @@ back, and the light functioned exactly the same as it did on battery power
 throughout this period. (For the last few weeks, we have mostly had cloudy,
 rainy days with only occasional bursts of sunlight.)
 
+![C-type backup batteries](./img/security-light-battery-unit.jpg)
+
 Other than the spiders, the only other main drawback is that it requires C-type
 batteries. As rechargable batteries of this type are hard to come by it meant I
 couldn't use a high-quality brand like Eneloop. I had to go for the least-worst