1. A system comprising: a data processing apparatus; and a computer storage medium encoded with a computer program, the program comprising data processing apparatus instructions executable by the data processing apparatus and that upon such execution cause the data processing apparatus to perform operations comprising: in response to an image insertion request specifying a request to insert image search results that are responsive to a search query into a list of general search results: determining dimensions of an image display environment in which image search results are to be displayed, the dimensions determined, in part, from a distribution of quality scores of images that are responsive to the search query and the dimensions defining number of rows of image search results that are to be displayed; determining an insertion score that defines an ordinal insertion position at which the image display environment is to be inserted into a ranking of the general search results that defines ordinal positions for each of the general search results, the insertion score determined, in part, from the quality scores of the images; generating a search results resource for displaying the general search results according to their respective ordinal positions and the image display environment at the ordinal insertion position on a client device; and providing the search results resource in response to the search query; wherein each of the image search results is a generated from a search of an image corpus and includes a thumbnail version of an image identified from the search of the image corpus and an address to retrieve the image, and each general search result is generated from a search of a resource corpus that is different from the image corpus and includes a snippet of text of a resource identified from the search and an address to retrieve the resource.