1. A system comprising: at least one processor; an index that associates, for a user, key items with images of screens captured from a mobile device used by the user, the screens having content generated by a mobile application executing on the mobile device, each image being associated with a mobile application that generated the screen captured in the image; and memory storing instructions that, when executed by the at least one processor, cause the system to: determine, using the index, that a first key item is responsive to a text-based query initiated at the mobile device by the user and that the first key item relates to a class of mobile application, identify responsive images, the responsive images being associated with the first key item in the index, rank the responsive images, including boosting respective ranks of responsive images generated by mobile applications in the class, provide a search result to the mobile device that includes a portion of highest ranked responsive images, wherein each portion includes an area of the corresponding image that displays the first key item, and responsive to receiving a selection of a first portion, open the mobile application that generated the first portion.