When you take a picture with a digital camera they can be really big, especially if you have a camera with 8, 10 or even 12 megapixels! These pictures often have too much detail for your printer.
If you want to include the image in a word, publisher or pages document these pictures can increase the file size by ridiculous amounts.
A top tip is to edit the picture in a program like Adobe's Photoshop and set the image size to one you need...