1. A method implemented by a data processing apparatus in data communication with an index of first and second resources, the first resources being resources that are different from the second resources, the method comprising: receiving queries for respective searches of the first resources; for each query, initiating a search of the first resources, and receiving, in response to the search, first search scores of the first resources, each first search score corresponding to a first resource and being indicative of the relevance of the first resource to the query; selecting, by the data processing apparatus, at least a proper subset of the queries and for each of the queries in the selected proper subset of queries determining a second resource score of the query, the second resource score of the query being determined independent of a search property ratio of the query; for each query in the proper subset of queries, comparing, by the data processing apparatus, the second resource score of the query to a second resource score threshold; for only each query having a second resource score that meets the second resource score threshold: initiating a search of the second resources for the query and receiving, in response to the search, second search scores of second resources in the proper subset of second resources, each second search score corresponding to a second resource and being indicative of the relevance of the second resource to the query; determining an insertion score that defines an ordinal insertion position at which second resource search results referencing second resources are to be inserted into a ranking of first resource search results referencing first resources; and generating a search results resource for displaying the first resource search results according to their respective ordinal positions in the ranking and the second resource search results at the ordinal insertion position; wherein: selecting at least a proper subset of the queries comprises selecting queries for which a confidence score of a search property ratio for the query does not meet a confidence score threshold, wherein the confidence score is a measure of accuracy of the search property ratio for the query; and the first resources are web page resources and the second resources are image resources.