We review evidence indicating that mental images, like pictures and percepts, depict rather than describe the represented content, and that visual images rely heavily on the same neural substrate as actual vision. Given this shared substrate, it is perhaps unsurprising that images also show certain sensory effects (e.g., sensory aftereffects) often associated with relatively low-level processes in vision. We argue, though, that images (like percepts and unlike pictures) are organized depictions, perceived within a perceptual reference frame that governs how certain aspects of the form are understood, and we consider some of the implications of this reference frame for image function. Evidence is also presented for a distinction between visual and spatial images, each with its own functional profile and its own neural substrate. Finally, we consider differences from one individual to the next in the vividness of mental images. We argue that, despite the skepticism of many investigators, vividness self-reports are interpretable and meaningfully linked to performance in at least some imagery tasks.