Adding alt text to images is important for 2 main reasons:

1)   Accessibility – visitors to your site that don’t see well and are having the page read to them need alt text to understand what the image is all about.

2)   SEO – the alt text is the only way a search engine knows what the image contains. It cannot ‘read’ an image by itself and relies on alt text. Additionally you can attract traffic from Google Image Search with well-optimized images.

By default in Blogger when you upload an image there is no alt text:

Here’s how you add it.

Go into Edit Post, then click on Edit HTML.
Look for the code that contains the info about the image – it will begin with <img :

Now simply add the following somewhere inside the img tag:
alt=”description of image”

Replace the words in quotes with something that describes your image – it can contain your keywords, but don’t be spammy about it.

Publish the post and voila, the alt text is available for search engines to see.