Dual-coding theory: Verbal and nonverbal information are stored separately in our memory, and simultaneously processing both verbal and nonverbal representations of an idea helps us remember better.
Article: Mental Imagery
Proposed by psychologist (and bodybuilder) Allan Paivio, the dual-coding theory of memory explains why processing both verbal and nonverbal representations of information at the same time helps us remember the information better.
According to dual-coding theory, our mind creates different representations for conceptual symbolic codes (like numbers and words) and perceptual analogue codes (like physical stimuli and images) and stores them separately in our memory. For instance, we can remember the concept "turtle" as both the word "turtle" and the image of a "turtle."
Both verbal and nonverbal representations can be used to recall information. Therefore, when we try to recall information, we can retrieve either the word or the image individually, or both simultaneously. This ability to code information in two different ways increases our chance of remembering it.
Images are generally remembered better than verbal information. This is also related to picture superiority effect, which is often used as a learning aid. However, when sequential order is required, verbal information is recalled better.
If you're interested in learning more about the dual-coding theory, you can read Conceptual Issues in Dual Coding Theory for its incompatibility with the computational approach to the mind (computationalism), and Dual Coding and Common Coding Theories of Memory for an alternative theory.