Google doesn't know how to interpret your images. In order to allow Google to understand your content, you can write an explicative text for each image. The ALT attribute allows you to add an alternative description for images of an HTML page. It is defined in the code of your page:
<img src="photo.jpg" alt="picture of my holidays in Miami" />
There is no standard length for the text of your ALT attributes. Avoid lists of keywords separated with commas and opt for a precise description of the content of your image. Ensure that the description matches with the content of your page.
Perfect ALT attribute: Malinois dog sat on a country road.
Go in your page analysis. Open the content tab and check the Images criterion. You can find a table listing all the images of your pages and their textual description.