Iphoto Book - Sizes of Photos


I am using Mac OSX (10.3.9) and iPhoto (5.0.4) and trying to creatte a book. When I try to fit photos into a selected page in a book I sometimes get an exclamation point in the corner. I know this means that the size of the photo is too large. I go into the edit mode and use the constrain button and select 4:3 (book) and click done so it saves. Unfortunately, it still doesn't fit into any page of the book. This does not happen to every photo. Do I need to change settings on my digital camera if my photos are to be used in an iPhoto book? I will also sometimes try and use photos I've found on the internet. I will also run into this same problem occassionally using photos from the internet.
Any ideas/thoughts would be great. I hate to omit photos in my book because of this.
Thank you,