13. In a digital media environment to facilitate access to collections of images using one or more computing devices, a system comprising; one or more processing devices; one or more computer-readable media storing instructions executable via the one or more processing devices to implement a caption generator configured to perform operations to automatically generate image captions using weak supervision data including: processing a target image for caption analysis via a convolution neural network (CNN), the CNN configured to extract global concepts corresponding to the target image; comparing the target image to images from at least one source of weakly annotated images to identify visually similar images; building a collection of keywords for the target image indicative of image details by extracting the keywords from the visually similar images as weak supervision data used to inform caption generation; supplying the collection of keywords indicative of image details to a recurrent neural network (RNN) along with the global concepts, the RNN configured to implement language modeling and sentence construction techniques for generating a caption for the target image; and generating the caption for the target image via the RNN using the collection of keywords to modulate word weights applied by the RNN for sentence construction.
13. In a digital media environment to facilitate access to collections of images using one or more computing devices, a system comprising; one or more processing devices; one or more computer-readable media storing instructions executable via the one or more processing devices to implement a caption generator configured to perform operations to automatically generate image captions using weak supervision data including: processing a target image for caption analysis via a convolution neural network (CNN), the CNN configured to extract global concepts corresponding to the target image; comparing the target image to images from at least one source of weakly annotated images to identify visually similar images; building a collection of keywords for the target image indicative of image details by extracting the keywords from the visually similar images as weak supervision data used to inform caption generation; supplying the collection of keywords indicative of image details to a recurrent neural network (RNN) along with the global concepts, the RNN configured to implement language modeling and sentence construction techniques for generating a caption for the target image; and generating the caption for the target image via the RNN using the collection of keywords to modulate word weights applied by the RNN for sentence construction. 14. A system as recited in claim 13 , wherein the at least one source of weakly annotated images includes a social networking site having a database of images associated by users with weak annotations indicative of low-level image details.