I had the same issue a couple of weeks ago.
First of all is important to be concern about the tinymce textarea will shows images and elements like a web browser will do, so you must to enter a right path for the href atribute image label (external published url o local url).
Once you ensure the url, you will change your configuration in order to avoid the tinyMce control will drop the image label from your code. I got this point with this configuration (using absolute paths):
tinyMCE.init({
...
//your initialization rules
...
// Drop lists for link/image/media/template dialogs
template_external_list_url : 'lists/template_list.js',
external_link_list_url : 'lists/link_list.js',
external_image_list_url : 'lists/image_list.js',
media_external_list_url : 'lists/media_list.js',
relative_urls : false,
remove_script_host : false,
convert_urls : true,
extended_valid_elements : 'iframe[src|frameborder|style|scrolling|class|width|height|name|align]'
});
I hope this code will help you!!