Here comes the Linux way (assuming
imagemagick is installed):
composite -dissolve 30% -gravity SouthEast -quality 100 logo.png in.jpg out.jpg
There is also ImageMagick for Windows, I guess.
Here comes a quick explanation of the command:
composite - Command we execute, part of the IM suite
-dissolve 30% - Give the watermark an opacity of 30%
-gravity SouthEast - Locate watermark in bottom-right (possible values here)
-quality 100 - Quite obvious, the JPEG quality for the output
logo.png - Your watermark, thanks to
-dissolve, doesn't have to be opaque
in.jpg - The original image that you wish to watermark
out.jpg - The filename of the result image
Now let's write a quick bash script to watermark every image in the folder
for i in originals/*; do
composite -dissolve 30% -gravity SouthEast -quality 100 logo.png "$i" "results/$(echo $i | cut -d '/' -f 2)"
Execute this in a folder containing your
logo.png as watermark, a folder
originals with your input images, and an empty folder called
results where the modified images will be saved in.