Magento 1.8.1 – Fix Broken Media WYSIWYG Image Browser

To use a separate domain, you need to take advantage of Store Views. In System > Configuration, the select box in the upper right-hand corner defaults is where you select what Store View you are configuring. The default is Default Config, which is applied to all store views and the admin (provided that the specific store view does not override the more general/global setting).

Therefore, if you set the Base media url here, that change will be applied to admin AND your store views. I would recommend selecting your store view and making the change there, as that will have no effect on the admin scope and thus it should not break the WYSIWYG editor.

Refresh your magento cache and flush everything