It makes it easier to find things when you're looking for them. If you keep your directory structure on your computer the same as on your server you can build your pages and view them from your computer or the web and they will be the same. As I mentioned earlier, coding the image is case sensitive. Mom's don't usually like to be capitalized anyway, they're friendlier than that. The same goes for the image extension.
Some programs save an image with a lower case extension and some with upper case.
If you type it in wrong it won't work. Check that if your image isn't showing up. Another common mistake is to incorrectly identify the path to the image. The final mistake to check when your image doesn't show up is often made when the image is uploaded to the server. Images must be uploaded in binary mode.