1. A computer-implemented method comprising: receiving, at a server computer system, a first digital image data and virtual tag data to be associated with a real-world object in the first digital image data, wherein the first digital image data is captured by a first mobile device, and the virtual tag data includes metadata received from a user of the first mobile device, wherein the virtual tag data received from the user of the first mobile device includes a virtual sticker selected by the user from among a plurality of different virtual stickers, each virtual sticker to convey different information, and the selected virtual sticker to be rendered over digital image data of the real-world object; generating a first digital signature from the first digital image data that describes the real-world object, wherein the generated first digital signature comprises a set of feature vectors generated from the first digital image data, each feature vector describing a neighborhood of pixels that surround an interest point for the real-world object in the first digital image data; in response to the generation, inserting in substantially real-time the set of feature vectors of the first digital signature into a searchable index of digital images as the set of feature vectors for the first digital image data are generated, the searchable index a searchable k-dimensional tree structure of feature vectors for a plurality of real-world objects; storing, in a tag database, the virtual tag data and an association between the virtual tag data and the first digital signature inserted into the index of digital images; receiving a second signature generated from a second digital image data captured by a second mobile device, the second digital image data includes one or more real-world objects; and transmitting the virtual tag data, received from the user of the first mobile device and associated with the real-world object in the first digital image data, to the second mobile device when a search of the index of digital images based on the second digital signature results in a match with the first digital signature.