Where are "import from iPhone" photos stored in the iPhone?
The Joe Friday Answer:


The picture was never recorded on your iPhone. That's the way the "Import" and Insert features work in Pages. 
You can get your picture from the Pages document easily:


Open the document in the Preview app 
control-click on the picture, then select Share, Add to Photos. From there, you can export if you wish. 



The Long Answer:

As I understand your question: 


You imported a picture you took on your iPhone to your Mac. 
You inserted that picture into a document you created on your Mac using the Pages app. 


Your question is ambiguous with respect to whether you "imported" the picture directly into the Pages app, or if you imported the picture from your phone to your Mac using some other (e.g. Photos) app. 

You may ask, "Why do I say your question is ambiguous?" Part of the reason is your wording (though certainly this subject is arcane), but the main reason is that the Importfrom iPhone item in the Pages menu is only available when you have selected an image that already exists in your Pages document. If your insertion point (i-beam, whatever) is at a blank spot in the document, then the menu item is Insertfrom iPhone. At least, this is the behavior that I see - your experience may differ. And so, if one uses Pages only occasionally (as I do), one might wonder if you got this mixed up with the Import feature in Photos (which always uses the word Import. But in any case...


If you "Imported" or "Inserted" directly from iPhone to the Pages document: The picture was never recorded on your iPhone. Sorry, but this is just the way the "Import" and Insert features work in Pages; apparently that's by design.  
If you imported from your iPhone to your Mac using the Photos app, then there is most certainly an option to delete from the iPhone, or to retain the image on your iPhone. If this is what you did, then you have selected the option to delete it after the import! And AFAIK, you don't get a "do-over" for that; the picture is no longer on your iPhone. 


And so, as to the location of your "missing" picture, it is in one of two places: 


If you imported the picture from your iPhone to your Mac using Photos, you can find it in your Photos app on your Mac, or in "iCloud" if you use that option. You can also export it from there if you like, again using the Photos app. 
It exists only in your Pages document. Again, not a big Pages user, but I'd guess there is some way to save or export the picture from the document. If you can't locate a better method, a couple of fall-back approaches are: 


Do a screen capture of the photo in your Pages document (commandshift5). 
Open the Pages document in the Preview app. From Preview, you can control-click on the picture, and then select Share, Add to Photos to add it back to the Photos app database - where you may have expected to find it in the first place!  :)