Previous research has shown that transparent idioms (e.g., paddle your own canoe) are generally easier for children to interpret than opaque idioms (e.g., paint the town red), results that support the metasemantic hypothesis of figurative understanding (M. A. Nippold, 1998). This is the view that beyond exposure to idioms and attention to the linguistic context, the learner analyzes the expressions internally to infer meaning, a process that is easier to execute when the literal and nonliteral meanings overlap. The present study was designed to investigate mental imagery in relation to the discrepancy in difficulty between transparent and opaque expressions. Twenty familiar idioms, half transparent and half opaque, were presented to 40 school-age children (mean age = 12;3 [years;months]) and 40 adults (mean age = 27;0) who were asked to describe in writing their own mental images for each expression. The participants were also given a written multiple-choice task to measure their comprehension of the idioms. The results indicated that mental imagery for idioms undergoes a developmental process and is associated with comprehension. Although school-age children were able to report relevant mental images for idioms, their images were less sophisticated than those of adults and were more likely to be concrete and to reflect only a partial understanding of the expressions. In contrast, the images reported by adults were more likely to be figurative. The findings suggest that the mental images people report for idioms may serve as a barometer of their depth of understanding of the expressions.