Duchastel 1978 described three roles for illustrations in text based on their functions:

1

• Duchastel (1978) described three roles for
  illustrations in text based on their functions
• an attentional role
• a retentional role
• and an explicative role.

2

• As Anglin, Towers, and Levie describe them,
• the attentional role relies on the fact that
  pictures naturally attract attention. (p. 765).

3

• The retention role aids the learner in
  recalling information seen in an illustration.
• (Anglin p. 765).

4

• the explicative role explains, in visual
  terms information that would be hard to convey in
  verbal or written terms. (Anglin p. 765).