Placing a signature or another sort of a watermark on the photos you have taken before publishing them in social networks is a good idea. This will protect your image gallery from unauthorized commercial use.
2JPEG can place your name or email address on the image. If you would like the watermark to be more visible, you can also place your logo or an avatar on your photos.
Here is the command showing how to automatically place the string (C) John Smith on all converted image files:
Also, you can use this command to place your logo on each converted picture or document:
If you like, you can combine several acts in the same command. As an example, you can scale images up to 1000 x 1000 pixels in size and place a watermark on output image files:
Complete description of 2JPEG command line parameters is available on this page.