ISO/IEC TS 20071-11:2012 applies to all static images that are used in any type of electronic document. It also applies to individual images within a slide show of electronic images.
ISO/IEC TS 20071-11:2012 does not apply to moving images (e.g. movies).
The guidance contained in ISO/IEC TS 20071-11:2012 is intended to be used by the person who creates content to be placed in an electronic document. There is no expectation that this person will have any additional expertise beyond understanding the contents of the document and why an image was chosen to be placed within the document.
While the main intent of the guidance within ISO/IEC TS 20071-11:2012 is the creation of text alternatives, the information identified in this guidance could be placed in the main document text, reducing the length of the resulting text alternatives. However, placing information in the main document text does not fully replace the function of having some text alternatives for each image.