The blogging bundle image_upload method creates a tag that doesn't validate. It is missing a slash before the closing bracket.
The diff with http://macromates.com/svn/Bundles/trunk/Bundles/ Blogging.tmbundle/Support/lib/blogging.rb is:
--- - 2007-02-18 18:45:49.000000000 -0500 +++ /dev/fd/63 2007-02-18 18:45:49.000000000 -0500 @@ -719,7 +719,7 @@ height_width = %Q{ height="#{height}" width="# {width}"} end end - print %Q{<img src="#{url}" alt="${1:#{CGI::escapeHTML alt}}"#{height_width}#{ENV['TM_XHTML']}>} + print %Q{<img src="#{url}" alt="${1:#{CGI::escapeHTML alt}}"#{height_width}#{ENV['TM_XHTML']}/>} end else TextMate.exit_show_tool_tip("Error uploading image.")