The cumuliform cloud rises though the freezing level but remains as supercooled water. At some point, depending on the abundance of nucleation sites, the liquid starts to change phase into ice. It is at that point, when the cloud starts to
glaciate, that it ceases to be a towering cumulus (as in your photo), and becomes a cumulonimbus. You'll start see some wispiness close to the top: ice crystals.
There always seems to be a perception among pilots that CU is "OK" and CB is "really bad". The working definition of a "CB" for CAT seems to be "something bigger than I'm prepared to fly through".
In reality, the convection cells go through phases where the potential for icing and turbulence varies greatly. If the TCU in your photo is penetrated above the freezing level, it could be an extremely bumpy and icy ride. Stay away from building cumulus. A dissipating CB can be quite tame by comparison.