How to create great alt text?

ALT Tags – If your browser cannot display an image from a website, then the ALT tag displays the description of the image as text. ALT image tags also make it possible for the visually impaired to understand the images on your website. The ALT tag should be only a few words describing the content of the image. ALT tags contribute to the keyword count on the Web page. So, using relevant images with appropriate ALT tags can increase the overall keyword count on your page.

 

Google Image publishing guidelines

  1. Don’t embed important text inside images
  2. Tell Google as much as you can about the image
  3. Give your images detailed, informative filenames
  4. Provide good context for your image
  5. Create a great user experience

The alt attribute is used to describe the contents of an image file. It’s important for several reasons:

<img src="puppy.jpg" alt=""/>

Better:

<img src="puppy.jpg" alt="puppy"/>

Best:

<img src="puppy.jpg" alt="Dalmatian puppy playing fetch"> 

To be avoided

<img src="puppy.jpg" alt="puppy dog baby 
dog pup pups puppies doggies pups litter puppies dog retriever 
 labrador wolfhound setter pointer puppy jack russell terrier 
puppies dog food cheap dogfood puppy food"/>

https://support.google.com/webmasters/answer/114016?hl=en