Elgg version? I thought we merged transparency handling long time ago in 1.8 branch.

18.16

Get image URL that's failing and comment out "Content-Type" header (in mod/file/thumbnail.php), to see if there are no warnings being injected into file contents.

Check also in your language files for whitechars before <?php or after ?> (should see them in output if that's the case)

Have you created any German language files on your own or modified any language files?

The problem could be caused by encoding of the German language files. Another users asked me for advice for quite the same problem just this week (Tidypics images not showing up). It turned out to be caused by saving the language files with BOM (Byte order mark characters). If you save language files you need to make sure to save them with encoding UTF-8 without BOM.

For the German language pack language files when not modifying them on your own the encoding is correct. If you modifyied them, replace them with the original unmodified versions. The same for other German language files for 3rd party plugins: either replace them with the original versions or disable any 3rd party plugins that have German language files temporarily to see if the problem with image thumbnails stops and until you fixed the language files (i.e. saved them without BOMs).

great thanks it's fixed by parsing all german traduction files with eliminate all carateres at begin of file  (before <!--?php) and at the end of file (after ?--> ) and save it with encoding UTF-8 without BOM.

elgg_get_entities_from_metadata(array(

  'type' => 'object',

  'subtype' => 'mysubtype',

  'metadata_name_value_pairs' => array(

      'name' => 'metadata_name',

      'value' => '%name_%',

      'operand' => 'LIKE',

      'case_sensitive' => false

  )

));

http://reference.elgg.org/engine_2lib_2metadata_8php.html#aec4b6f0b9565e3554acb9b39ef34a2ac

great thanks