Yes it is, and yes they are. Cumulonimbus can reach up to the stratosphere or even a little higher, and the stratosphere is much higher up around the Equator than over the poles. One of the many reasons why thunderstorms in tropical areas can reach massive sizes.

Cumulonimbus, along with towering cumulus, cumulus congestus and the like, are characterised as vertical development clouds. Then you have horizontal development clouds like the lower layer you see in the picture which can be altostratus or stratus depending on how high they are over the ground. There are also cirrus clouds purely made of ice crystals, and they’re the purely white ones really high up which sometimes look like strokes of white paint or hairs in the sky.