CiviCRM Converting HTML tag to Entity
I'm assuming you're using the default mailing Ux, not mosaico

The default editor is wysiwyg, if you want to put html tags, click on the view source icon, and paste the html code you want. 

You can switch back to wysiwyg to see the result.

As for the images, be aware that quite a few recipients aren't going to display the image (for privacy reasons), so you should consider putting an alt text and dimensions (width and height) on your img tag