You can keep a copy of your web pages and images on your hard drive using the same directory structure as you use on your web site, and the URLs will work on both without any changes. You'll have an easier time of it if you move your site to another host. If you're having problems with an image not showing up on your page, the browser probably couldn't find it. Remember to check case sensitivity. Some FTP programs may change the case when it uploads the file.
All recent browsers support .gif and .jpg images and many support .png.
You should avoid using these since other browsers don't support them. Uploading in ascii may cause the file to become unusable. The alt attribute Use the alt attribute to assign a short description to the image. In all browsers, the alt text is displayed in place of the image until the image finishes downloading. In all browsers, the alt text remains displayed if the image cannot be found or if the visitor is surfing with images disabled.