Watermarking

The following free, cross-platform software can be used for visually watermarking your images:

• aPhoto
• BorderMaker
• BRIC
• darktable
• digiKam
• EasyImageSizer
• Fotoxx
• G'MIC
• Imager
• ImageMagick
• JACo Watermark
• Phatch
• XnConvert
• XnViewMP

Apply a hidden watermark to your images:

• G'MIC

  See the "Fourier watermark" tool in the "Spectral filters" menu.

  Reference: -watermark-fourier (Document)

• Wavelet Based Steganography and Watermarking (Document)