Ooh, where do we start?
Firstly, when the publishers say the photos must be 300dpi, it's almost certain that they don't know what they really mean by that.
Secondly, any dpi figure embedded in your photos is totally meaningless.
With me so far? Right, now we can move on.
The only thing that really matters is how many pixels you have in your photos. High quality printing usually runs at about 300 pixels per inch (ppi). It follows that if you want your image to print at 4 inches across you need 1200 pixels; 6 inches requires 1800 pixels; and so on.
So what you need to do is:
(1) Ask the publishers what size they want to print the photos.
(2) Resize them to appropriate dimensions based on 300 pixels per inch.
(3) Edit the dpi / ppi value in the metadata to say 300. (This doesn't achieve anything in practice but may keep the publishers happy.)
How to do (2) and (3) depends on which software package you use. The dpi / ppi value is usually an option on the resize dialogue or on the save/export dialogue. If in doubt, ask.
