Some aritsts afraid of unauthorized use of their works. They want us add watermark to their works which can let them easily track where those picture comes from.
Topic on Project:Support desk
Note mediawiki already adds invisible metadata saying things like File source: http://commons.wikimedia.org/wiki/File:%25_of_U.S._households_connected_by_age.png, which is readable by programs like exiftool. (This is only on the thumbnails. MediaWiki doesn't touch the original image).
You can also use $wgCustomConvertCommand in order to set a custom command for making thumbnails, which can be used to add a visible watermark.
I got it! but the only problem is that gif can not be re-sized. I am working on it.
What do you mean with "gif can not be re-sized"? Gif thumbs are not animated?
There is only few words say "thumb not success"