What does that.... alt="" ...do in the code
If your image doesn't displays , anything written in alt tag inside double quotes will be displayed instead of image
In case due to some problem the system is unable to show the image It will show an alternative text in place of the image.... It's actually not very important....
alt attribute is useful if the user internet disconnects or weakens or even their is a mistake in src value for any reason, any thing you typed insie alt it will be displayed instead of image...