You can remove watermarks or any unwanted portions from your picture easily with GIMP. There is a great plug-in for this purpose named “Resynthesizer”. With the help of this plug-in, you can remove watermark in few seconds!
First you need to install the “Resynthesizer” plug-in. Open you terminal and run the command-
sudo apt-get install gimp-resynthesizer
Windows users can also have complied version from its website.
Now open the picture with GIMP and simply select the portion you want to remove. From the menu select Filters > Map > Resynthesizer. A window will appear, click Ok button and save the picture. You will see your unwanted portion is removed. Try it!