So this is What you called Alt Tag.
Alt tag is where you will put the description of the photo.
Basically the HTML code for an image is
So we need to add an alt tag to this so search engine ca read whats the photo all about.
The HTML code for an Image with Alt Tag is
<img src="image/picture1.jpg" alt="picture description" />
Take Note that done overuse Alt tag. Just put a brief description not exceeding to 12 words or less.
Code for Alt Tag | How to use