In order to add the title attribute to links and images use the code section of the insert function.
<h2>Link Titles</h2><p>The title attribute specifies extra information about an element. The information is most often shown as a tooltip text when the mouse moves over the element.</p><a href="https://productforums.google.com/forum/#!forum/sites" title="Go to the Google Sites Help Forum">Help Forum</a>
To get a usable URL for a Google Drive image you need to get the image's ID and append to this URL:
<img src="https://drive.google.com/uc?export=view&id=1y8Y8R1Daf5kbTfNQyf6h_hY_EfBmw70VYg" width="70%" title="At Google">
When inserting an image the regular way we are able to add alt text. Perhaps we can send feedback and suggest that they add the title option.