When you embed an image to an ezoe field you get this warnings in debug:
Warning: PHP: E_WARNING
Oct 30 2012 15:51:44
json_encode(): Invalid UTF-8 sequence in argument in /var/www/apache2php53/ezp5/ezpublish_testsystem/.run/extension/ezjscore/classes/ezjscajaxcontent.php on line 158

Steps.
1. Create an Article
2. On the body field click insert/edit image
3. Upload an local image
4. Before clicking "OK" verify the errors in the New <embed> tag window