The theoretical difficulties in indexing images include: 1) images do not satisfy the requirements of a language whereas textual materials do 2) images contain layers of meaning that can only be converted into textual language using human indexing 3) multi-disciplinary nature of the images where the terms assigned are the only access points. Theoretical foundations for image indexing consist of distinctions between classes of terms including ‘of’ and ‘about’, syntactic and semantic, specific and generic, and answers to the questions ‘who?’, ‘what?’, ‘where?’, and ‘when?’. Content-based indexing can be used to generate terms for the color, texture and basic spatial attributes of images. Image searchers use textual descriptor search terms that require human description-based indexing of the semantic attributes of images.