BINGO!!! Finally got it !

I used Alternative Solution with dvipng http://meta.wikimedia.org/wiki/Running_Mediawiki_on_Windows#Alternative_Solu...

But moreover, in the includes/math.php for this solution http://meta.wikimedia.org/wiki/Help:Running_MediaWiki_on_Windows/math-dvipng...

I changed var $_dvips_path = 'dvips.exe';

to var $_dvips_path = 'dvipng.exe';

Beforehands the fatal problem was, that my dvipng.exe was not working well - no error written, but it did simply nothing. I changed from TeXlive to MiKTeX and it's ok...