16. A computer implemented method performed by a data processing apparatus, comprising receiving, in response to input by a user in a user device, content comprising text; receiving a candidate set of image suggestions, the candidate set comprising images that may be submitted to a user taken from one or more collections of images; for each of the image suggestions in the candidate set of image suggestions determining whether the image suggestion satisfies an association criterion that defines the image suggestion as being associated with the content, the determining comprising; determining an association function for calculating an Illustration Index, comprising a score that measures the level of association of the image suggestion to the content; in response to determining that the image suggestion satisfies the association criterion, including the image suggestion in a final set of image suggestions; and; in response to determining that the image suggestion does not satisfy the association criterion, excluding the image suggestion from the final set of image suggestions; and providing the final set of image suggestions for display on the user device.
17. The method of claim 16 , wherein the final set of image suggestions are ordered based on the Illustration Index.