A picture of a dog, or an actual dog itself, is a non-verbal stimulus. It is a visual stimulus that is not the result of prior verbal behavior.
A physical energy change capable of affecting an organism’s sensory receptors that has the following features:
It has a specific form or pattern which, as a unit, has controlling effectiveness
It is the result of verbal behavior
Modality: auditory, visual, etc.
Dynamic characteristics such as size, intensity, etc.
Function: discriminative, reinforcing, punishing, etc.