If you are supplying images for inclusion in your book they need to be the right type of image and the right size.
Photos and illustrations should be provided in jpeg format - that means the files have an extension like so "filename.jpg"
You also need to ensure that the photo is large enough i.e. is of a sufficient resolution to print well. If the file is too small it will "pixelate" when enlarged.
Our rough guide is that the jpg file should be at least 3MB in size to print well in a book. If a colour image is to print as large as an A4 page in something like a cookbook, the image might need to be as large as 24 Mb in size. Certainly if your images are sized in the Kilobyte range e.g. 345Kb it is going to be far too small.
If you want to get technical here's a website that will explain everything: Resolution and file size