X-ray picture vs skiagraph

X-ray picture and skiagraph both are nouns.

Word	Noun	Adjective	Verb	Adverb
X-ray picture	Yes	No	No	No
skiagraph	Yes	No	No	No

As nouns, skiagraph is a hypernym of X-ray picture; that is, skiagraph is a word with a broader meaning than X-ray picture:

X-ray picture: a radiogram made by exposing photographic film to X rays; used in medical diagnosis
skiagraph: a photographic image produced on a radiosensitive surface by radiation other than visible light (especially by X-rays or gamma rays)

Other hypernyms of X-ray picture include radiogram, radiograph, shadowgraph, skiagram.

X-ray picture (noun)	skiagraph (noun)
a radiogram made by exposing photographic film to X rays; used in medical diagnosis	a photographic image produced on a radiosensitive surface by radiation other than visible light (especially by X-rays or gamma rays)