The platform automatically adds alt texts or other data attributes (depending on which module is being used) to images to make them more accessible. These texts are taken from the Title field which are set in the Asset Manager.
If you make use of Inline images in a text block using the 'image' icon, you can set it yourself by filling out the 'change image text alternative' input field:
An alt text is used when a browser can not display the image or for accessibility reasons.