The recent revolution of digital camera technology has resulted in much larger collections of images. Image browsing techniques thus become increasingly important for overview and retrieval of images in sizable collections. This paper proposes CAT (Clustered Album Thumbnail), a technique for browsing large image collections, and its interface for controlling the level of details (LOD). As a preprocessing, this new system applies tree-structured clustering to images based on their keywords and pixel values, and selects representative images for each cluster. When a user specifies one or multiple keywords, CAT extracts a branch of the tree structure that contains clusters described by the user-specified keywords. A hierarchical data visualization technique is developed to display the tree structured organization of images using nested rectangular regions. Interlocked to the zooming operation, CAT selectively shows representative images while zooming out, or individual images while zooming in.