Caution icons while making a book in iPhoto 5

Question

hello. I've been working on two books (large hardbound), and both book layouts are littered with caution icons. For one of the books, it's for all of the images. For the other, they're scattered around. Most of these pictures are at least 1000x1000 pixels. Some of them are bigger, and some of them still get caution icons. What is the minimum pixelage that the images for the large hardbound book need to be in order to be considered acceptable? How does iPhoto determine whether an image will be low quality or not? How accurate is iPhoto's low quality image assessment?
Tried repairing disc permissions, and restarting. Also tried importing the photos to a different user and creating the book again. No difference.
Both books look fine in layout mode. Pretty crazy good, actually. I downloaded prefsetter, used it with iPhoto prefs and boosted the book dpi to 300, so that should improve the print quality. I thought about just sending the one to get printed to see what happens, since it's ready to go. Not quite sure I want to risk $35 bucks just yet, so I thought I'd look for some guidance from the more knowledgable.
Thanks in advance,
Ben

Answer 1

As luck would have it, I happen to have Graphic Converter as it came with my mac (yay). Unfortunately, it seems that every image I want to use for both these books is 72x72 dpi. I find that really disturbing. So are all my images worthless? Why does photoshop and both my digital cameras generate images in 72x72 dpi by default if it's inevitably such a low print quality?
I'll be honest. I've been poking around GC for about 30 minutes, and I have no idea how to convert anything to anything, let alone raise the DPI. I'm pretty discouraged at the moment. In photoshop, it seems that the default settings are set for 300 dpi "print resolution", and 72 dpi for "screen resolution." I can't find anything in GC that will confirm this. Just standard PPI, which is 72. So are the photoshop settings okay, or should I change them?
Could I use prefsetter to change the iPhoto book prefs to 72 instead of 300? Would that work for me or against me?
Any other thoughts on this would be most appreciated. I'm feeling a bit overwelmed, however, everything you've told me so far has been very helpful regardless.
Thanks again,
Ben