To take screenshots with mplayer, you need to first run mplayer with a special flag.

mplayer -vf screenshot movie001.mp4

Then once the movie is playing you can screen shot any frame by hitting the 's' key. mplayer will name the files starting with shot0001.png.

When I first tried …

I decided to try and start signing my pictures.

composite -watermark 15% -gravity southeast sign-1.png $i $i;

It needs more tweaking but that will do for now.

To convert all files in a directory:

for i in *.JPG; do
        echo -e "$i... \c";
        composite -watermark 15% -gravity southeast sign-1 …

Three ways to invert an image:

convert black-white.png -negate white-black.png
convert upside-down.png -rotate 180 rightside-up.png
convert upside-down.png -flip rightside-up.png

I already posted about image magick before but since this time I actually needed to use it, I found the "for" statement to work better than find.

$ for i in *.jpg; do echo "$i"; convert "$i" -resize 600x "$i"; done

Edited:

$ for i in *.JPG; do echo -e "$i...\c …

To batch resize images using image magick:

find . -name “*.jpg” -exec convert {} -resize 800×600 {} \;