Anna, it's ok that the photo says it's 72 dpi if the pixel count is 5184x3456. It becomes 300 ppi when you place the photo into your layout which is 300 ppi.
If you open a photo on its own in Photoshop and it says it's 72ppi, just change that to 300 ppi for printout by itself (keep "resample image" UNCHECKED). You'll see the photo dimensions change.
For example, my photo (downloaded from my son's FB account) comes out as 720 x 540 ppi, or 10 x 7.5 inches, at 72 ppi. If I change the resolution to 300 ppi I will see the dimensions change to 2.4 x 1.8 inches, while the pixel count remains the same. So, I should not try to print that photo out at 8x10 inches because the resolution doesn't support it--the pixels are not there to begin with. But with your photos, you've got the almost 18 million pixels to work with that came out of your camera (18 megapixels!).
So stop sweating about the 72 ppi photo as long as you're working with a 300 ppi page layout. It'll be larger than you need, and you can safely scale it down to fit, or use it to fill the whole 12x12 page if you want!