Combining different input modalities beyond text is a key challenge for natural language processing. Previous work has been inconclusive as to the true utility of images as a supplementary information source for text classification tasks, motivating this large-scale human study of labelling performance given text-only, images-only, or both text and images. To this end, we create a new dataset accompanied with a novel annotation method - Japanese Entity Labeling with Dynamic Annotation - to deepen our understanding of the effectiveness of images for multi-modal text classification. By performing careful comparative analysis of human performance and the performance of state-of-the-art multi-modal text classification models, we gain valuable insights into differences between human and model performance, and the conditions under which images are beneficial for text classification.