Images can be drawings, micrographs, photographs, structures, sequences, graphs, and more.
You need to provide references for any image, and that includes images from the web.
Types of sensors.
Source: Infrared sensors for counting, classifying, and weighing vehicles. (1990) Joseph Garner, et al. Center for Transportation Research, Texas. Page 2.
Enrollment by sex.
Source: National Science Foundation; Division of science resources studies.Graduation enrollments climb in science, engineering and health. (1990). Washington D.C.