The problem is that Magento creates a dynamic link with an admin path in it. See line 8 in your error: /index.php/admin/cms_wysiwyg/directive/___directive/e3ttZWRpYSB1cmw9Ind5c2l3eWcvcmd0LXNsaWRlci1pbWcwMS5qcGcifX0,/key/ceed8184f5f336aafcc307f8623aff45/
Since this link does not have a known image extension at the end, I assume that the error you received has occurred.
The solution is to change only one parameter in admin:
System -> Configuration -> General -> Content Management -> Use Static URLs for Media Content in WYSIWYG for Catalog to Yes
I found that this option is marked in /app/code/core/Mage/Cms/Helper/WYSIWYG/Images.php on line 180 (in Magento 1.7.0.2) and depending on whether static or dynamic links are generated.
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