1. A computer-implemented method for identifying pictures in documents, comprising: at a computer system including one or more processors and memory storing one or more programs, the one or more processors executing the one or more programs to perform the operations of: receiving an image representing a page of a document; identifying a plurality of captions in the image; identifying a respective portion of the image that includes non-text graphics; determining that the respective portion of the image includes a plurality of distinct pictures based at least in part on the quantity and location of the previously identified plurality of captions in the image; and storing metadata tags for pictures in the plurality of distinct pictures, wherein a respective metadata tag for a respective picture includes caption text for a respective caption associated with the respective picture; wherein determining that the respective portion of the image includes the plurality of distinct pictures based at least in part on the quantity and location of the previously identified plurality of captions in the image includes: determining a preliminary grouping of pixels in the respective portion of the image into one or more groups of pixels; generating a refined grouping of pixels that groups pixels in the respective portion of the image into one or more groups of pixels, wherein the refined grouping of pixels corrects for under-segmentation and/or over segmentation in the preliminary grouping of pixels; and identifying the plurality of distinct pictures based on the refined grouping of pixels.