Imagemagick

Install Imagemagick, a powerful command line image tool, with Homebrew in my iMac.
1. install XQuartz first, download file from http://xquartz.macosforge.org
2. brew install imagemagick --with-x11
3. convert logo: logo.gif; display logo.gif

I use it for raising the edge of my photo for some 3D effect.
$ convert -raise 5x5 photo-in.png photo-out.png

Then I will use it to make a stamp image as watermark:
$ convert -size 360x100 xc:black -font Monotype-Corsiva -pointsize 72  -fill black -annotate +24+64 'Shi Zhilong' -fill white  -annotate +26+66 'Shi Zhilong'  -fill transparent  -annotate +25+65 'Shi Zhilong' 1.png

$ convert -size 360x100 xc:transparent -font Monotype-Corsiva -pointsize 72  -fill black -annotate +24+64 'Shi Zhilong' -fill white  -annotate +26+66 'Shi Zhilong'   -fill transparent  -annotate +25+65 'Shi Zhilong' 2.png

$ composite -compose CopyOpacity 1.png 2.png Stamp.png