I was having a problem embedding a slide show from Picasa into my BlogEngine.net editor. Basically, the editor kept stripping the embed html.  The trick is to edit \admin\tinyMCE.ascx file.  You need to modify the extended_valid_elements entry. Apparently this is used to define valid html tags for the tinyMCE editor. Here are my additions in red:

extended_valid_elements : "hr[class|width|size|noshade],font[face|size|color|style],span[class|align|style],

script[charset|defer|language|src|type],code,iframe[src|width|height|frameborder|name|style],

embed[type|src|width|height|flashvars|pluginspage]",

Have fun!