Why are they 'nude photos'? [duplicate]

Solution 1:

Collocations modifying photo often don't refer to the photo as a physical object. They instead refer to the subject of the photo, or what's depicted in the image.

To demonstrate this, here are the most common collocations for ____ photo according to the Corpus of Contemporary American English. I have bolded the ones that describe the image (source, subject, or whatever) and italicized the ones that describe a property of the physical photograph. Other results are left unchanged. Numbers describe frequency within the corpus.

• these (434)
• family (379)
• two (316)
• take (243)
• color (234)
• AP (225)
• those (222)
• taking (180)
• digital (168)
• old (160)
• scene (147)
• three (135)
• nude (129)
• framed (124)
• satellite (122)
• black-and-white (113)
• took (112)
• aerial (91)
• four (87)
• five (84)
• snapping (82)
• wedding (63)

Out of these results, a family photo is understood to be a photo of a family, just as a wedding photo is understood to be a photo of a wedding. Similarly, English has other constructions, like nude photo. The same positioning can also describe provenance (AP photo, satellite photo, aerial photo), quality of photo (black-and-white photo, color photo), physical status (framed photo, digital photo), and so on.

Hearers understand nude photo to refer to what's in the photo because of established usage. Also, the idea of a photo in the "nude" does not make much sense, so the physical interpretation of the object is unlikely. Many media objects have this quality; a ___ book can refer to either the physical object (big book, hardcover book) or to a quality of the text inside (a sad book, a scholarly book). A bit of logic and some arbitrary usage rules determine how people interpret collocations involving media objects.

Solution 2:

nude ADJECTIVE
...
1.1 [attributive] Depicting or performed by naked people.
‘she won't do any nude scenes’
Lexico