There is a bug concerning Rich Text & labels (css class)
A label applied to an img is lost when an existing document is edited.
Steps to reproduce:
Add a label to an image in a Rich text field.
Save and publish the doc.
Edit the document again
The label applied to the image before is lost, so if you save and publish the doc you'll loose the information.
In the following thread,
Phil states that it is not possible to add labels to images bot this it is not true.
Here is the model to achieve this:
"multi": "paragraph, strong, em, hyperlink, image, list-item, o-list-item",
"label": "Rich description",
"labels": ["cms_text_align_left", "cms_text_align_center", "cms_text_align_right"],
"placeholder": "Rich description"