Motor as an adjective

Don't worry about this, I'm pretty confident that it is only really being used as an attributive noun here.

It's a boat with a motor (noun)., 'Motor' cannot be modified by 'very' and has no comparative or superlative forms. So it's a noun. However, does that matter? Its function is to modify another noun. As David wrote, don't worry about it (unless you have one of those teachers who thinks there is some point in labelling parts of speech).

(later) I have just checked what some of the dictiona

Sadly, I have to edit a book on grammar for children and am finding that the original text is riddled with potential errors. I'm also trying to determine whether 'animal' in 'we were looking at animal images that had been painted...' is an adjective as it's presented as being such, and I know it exists as an adjective. Does it act as one here, though? My instinct is no, it's an attributive noun

I agree with you, but you' need to discuss terminology with the author and publisher. I don't think it's up to a proofreader to change the terminology of one school of grammar to that of another.

The publisher has thrown it back to me, having made errors in their previous text that I picked up, and the author isn't a professional lexicographer...
To me, 'animal' in its adjectival form refers to something physical (e.g. animal instincts) or something deriving from an animal source. An 'animal image' on a painting seems to refer to an image of an animal, as opposed to an image that is