The GIMP or GNU Image Manipulation Program -- a free, open-source software program that can be downloaded from gimp.org -- has many of the same features as a professional, proprietary image editing program, and if a watermark is created on a layer within an image, you can delete the watermark layer by using GIMP. However, if the watermark is part of the image layer itself, the only way to remove the watermark is to edit the image.
Open the image in GIMP. Click the "File" option on the top navigation bar, and then click "Open as Layers." The File Open window launches.
Navigate to and click on the watermarked image. Click "Open." If it's layered, the image opens as layers in GIMP.
Click on the watermark layer of the image in the GIMP working canvas.
Press the "DEL" key. The watermark layer is deleted.
Click "File" option and then click "Save" to save your changes.