That's in interesting question, and I don't know the answer. (It would be easy to figure it out for some...put some photos on the iPhone and the iPad and use on the utility apps that can get into the iDevice file system to compare sizes).
But to speculate, I'd guess the photos are the same size since the iPhone 4 screen resolution is pretty close to iPad screen resolution.
My buying recommendation: Because this is a concern for you, just get the 64GB iPad, unless $100 is going to break your bank. I have a 64GB iPad and it's only about half full. But I think of that empty space as potential. And especially with the new iPad's cameras, you could fill that space pretty quickly with pictures and video that you shoot yourself and edit in iMovie, or with music that you create or record.