You need Alt Text and Title Text on Every Image for Better SEO.

You should be using Alt Text on your Images. 

Alternative Text, Alt Text, or Alt Tag, is the text that aids accessibility by describing the images which appear in your posts.  The  Alt Text should be as descriptive and specific as possible. The purpose of alt text is to convey what is in the image and why it appears in the post. The purpose of alt text is not to convey the keywords for which you wish the image or post to be ranked. In fact, using it as another place to stuff keywords may be a negative SEO factor.

The Alt Text is not necessarily the same as the Caption or the Image Title. 

When you insert a photograph to your page or post, Google Blogger does not populate the Caption, Image, or Alt Text fields.  The "Caption" is most frequently used and appears on the main screen when you insert an image.  However, the "Alt Text" and "Title" fields for the image are hidden under the "Gear" icon.


Callout pointing to 'gear' icon on Google Blogger Image Screen in Blogger Editor
Pointing out the 'gear icon' when adding or editing an image on a post.

Screenshot of Google Blogger Alt Text and Google Blogger Image Title fields.
'Alt text' and 'Title text' fields in Google Blogger

What is the 'Title Text' for an Image in Google Blogger? The Title Text is the "on hover text" that displays when a desktop user hovers the mouse cursor over an image on your blog or post.

Screenshot showing Image Title appearing on hover in Google Blogger.
The Image Title appears on hover over images in Google Blogger.