Watermarking

The following free, cross-platform software can be used for visually watermarking your images:
 * aPhoto
 * BorderMaker
 * BRIC
 * 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)