I've changed this line in .htacces

I can't check this gdlib now, but I've also installed on the same server elgg 1.6.1 for test purpose and it is working:

http://testelgg161.ibornholm.com/

One of your PHP files is sending a UTF8 BOM character.  The non-binary pages can handle this fine in the browser, but images and other binary files can't.  Have you used a Windows machine to edit any of the files?  I know Notepad will mess up PHP files with UTF8 strings, but I'm not sure what Windows editors are best.  There's a good bit of info on this site and Google and how to deal with BOM characters.  Unfortunately, it can be a bit of a hunt to find which file it is :-/

Thanx for clue. Fortunately I know which file I've saved in Notepad. And fortunately this was only one file - polish translation. I have removed it and now images works fine. Thank You again. You are the best! :-)