Options for adding lower gearing for bike with Shimano 105 drivetrain
There are a couple of parameters you need to pay attention to:


The rear derailleur's wrap capacity (how much chain it can wrap up)
The biggest gear the rear derailleur can fit under.


With your current setup, your derailleur needs to wrap 41 teeth of chain. It looks like you've got this derailleur, which can wrap 37 teeth and fit under a 32-tooth gear. So you're already beyond capacity in terms of wrap, but if you don't cross-chain, it's probably not a big deal. You could probably get away with a 34-tooth bottom gear by adjusting the B screw on the derailleur (the one pointing straight back, under the hanger) to push the derailleur down.

The current generation of 105 medium-cage derailleurs is designed to fit under a 34-tooth bottom gear and handle 39 teeth of wrap.