How to get tinymce to keep absolute image paths?

I need tinymce to keep absolute image paths, not strip them. I tried the advice in this post about setting these in the tinymce config:

<config key="relative_urls">false</config>
<config key="convert_urls">false</config> 

http://blog.ovesens.net/2011/01/umbraco-tinymce-converting-absolute-to-relative-url/

It doesnt seem to work though.