We study several problems in image retrieval systems. These problems and proposed techniques are divided into three parts. Part I: This part focuses on robust object representation, which is of fundamental importance in computer vision. We target this problem without using specific object models. This allows us to develop methods that can be applied to many different problems. Three approaches are proposed that are insensitive to different kind of object or image changes. First, we propose using the inner-distance, defined as the length of shortest paths within shape boundary, to build articulation insensitive shape descriptors. Second, a deformation insensitive framework for image matching is presented, along with an insensitive descriptor based on geodesic distances on image surfaces. Third, we use a gradient orientation pyramid as a robust face image representation and apply it to the task of face verification across ages. Part II: This part concentrates on comparing histogram-based descriptors that are widely used in image retrieval. We first present an improved algorithm of the Earth Mover's Distance (EMD), which is a popular dissimilarity measure between histograms. The new algorithm is one order faster than original EMD algorithms. Then, motivated by the new algorithm, a diffusion-based distance is designed that is more straightforward and efficient. The efficiency and effectiveness of the proposed approaches are validated in experiments on both shape recognition and interest point matching tasks, using both synthetic and real data. Part III: This part studies the thumbnail generation problem that has wide application in visualization tasks. Traditionally, thumbnails are generated by shrinking the original images. These thumbnails are often illegible due to size limitation. We study the ability of computer vision systems to detect key components of images so that intelligent cropping, prior to shrinking, can render objects more recognizable. With this idea, we propose an automatic thumbnail cropping technique based on the distribution of pixel saliency in an image. The proposed approach is tested in a carefully designed user study, which shows that the cropped thumbnails are substantially more recognizable and easier to find in the context of visual search.