Watermarks are embedded into images to establish copyright in the first place. Removing the watermark from a copyrighted image and distributing the image violates some rights and laws. It's OK to try to remove the copyright as long as you don't distribute the picture or use it for commercial purposes but still it shouldn't be done.
You can try a graphical editor to remove text from images. For this purpose you can use Gimp or Adobe Photoshop as being the most used applications used in graphical editing.