With CSS Pro, you can edit an element's HTML attributes and inner HTML content.
To do that, click on the "HTML" tab:
You can add, edit, and remove HTML attributes on this tab.
The src attribute on img elements get a special interface that shows you a preview of the image and also lets you change that image. You can drag and drop an image there, or paste the image file, or click on the image preview to select an image from your computer.
The image is then converted to a base64 URL:
You can also edit the inner HTML when the element is not a img.