1. A non-transitory computer-readable storage medium that stores program instructions computer-executable to implement: determining a plurality of semantic category scores for a digital image via application of a corresponding plurality of classifiers; automatically determining a semantic category profile for the image based on the plurality of semantic category scores, wherein the semantic category profile characterizes semantic content of the image, and is useable to perform semantic based operations with respect to the image; and performing a semantic based operation wherein the semantic based operation comprises: a search operation based on a semantic similarity measure for a plurality of semantic category profiles for a plurality of digital images, wherein the plurality of semantic category profiles includes the semantic category profile for the image; or a keyword operation based on a semantic similarity measure for a plurality of semantic category profiles for a plurality of digital images, wherein the plurality of semantic category profiles includes the semantic category profile for the image.