Categorization is crucial to human perception and cognition. Without the abilities to categorize, people would not be able to deal with the multitude of things they perceive daily. Past research has shown that categorization also facilitates effective and efficient browsing of both text and image documents especially when users have no specific document in mind but only some idea of what they are looking for. However, the question of what types of attributes should be used to index categories/groupings of images is not fully answered. Knowledge about how people categorize images and assign names to categories may provide some clues to help answer this question. Results of this free sorting study, as well as other research on the nature of categorization and search requests by image users, point to the fact that interpretive attributes are better candidates than perceptual attributes for indexing categories/groupings of images.