The search engines (Google) only read the Alt Text and the file name of the image as they cannot interpret the image itself. This makes the Alt tag an important ranking factor with respect to image optimization and in particular the overall SEO.
ALT Tags best practice:
- Describe the content of the image to the users as short and precise as possible
- Use the most important keywords if possible (primary and secondary)
- Avoid keyword stuffing - thus a string of keywords
If possible name the image file using a primary or secondary keyword.
Good example:
Image file: puppy.jpg
Alt: Labrador puppy plays with ball on a lawn
Bad example:
Image file: puppy.jpg
Alt: Puppy
Avoid completely:
Image file: puppy.jpg
Alt: Dog, puppy, puppies, Labrador puppy, cute puppy, Labrador