You will first arrange the instructional stimuli (pictures) in front of the learner. When you place the stimuli in front of the learner, any variable could affect the learner’s response. Be careful about the ordering and location of each stimulus because unusual or unintended features can impact responding (e.g., the correct picture happens to be closer or always in the center).
Note that some advanced learners may benefit from learning with a messy array of stimuli. This is more typical in the natural environment.