I am using the latest version of Omeka and have included the img element in the list of allowed HTML elements and the src and alt attributes. However, Omeka still seems to be filtering these attributes and elements. I am using the HTML editor button. The only way around this seems to be unchecking HTML filtering, putting the full img tag in with the src and alt attributes like
<img src="http://www.xxx" alt="imagedescription" >
After the code is in, I check the filter in the HTML settings again and the tag and attributes are not filtered and remain on the public page and in the HTML editor.
List of Allowed HTML elements
List of HTML attributes