Methods, systems, and apparatus, including computer program products, for ranking images are disclosed. An image search subsystem generates an adjustment factor representative of a quality measure of an image relative to a search query. The quality represents a relevance of the image to the query. The adjustment factor can be computed based on relevance data for the image to the query and image similarity data representing a relative similarity between the image and other images relevant to the query. The relevance data can be based on user actions in response to the image being included in search results for the query. The adjustment factor can be scaled based on whether the relevance data and the image similarity data both indicate that the image is relevant to the search query. A relevance score is computed based on the adjustment factor (e.g., a product of the adjustment factor and relevance score).