alt attributes, also known as alt text, are what browsers will display if they fail to load the image. alt attributes are also important for blind or visually impaired users to understand what an image portrays. And search engines also look at alt attributes.
In short, every image should have an alt attribute!
You can add an alt attribute right in the img element like this:
<img src="www.your-image-source.com/your-image.jpg" alt="your alt text"/>