What symbols represent Zazen? I am looking for symbols which specifically represent Zazen, or, more generally the act of meditating.
The symbol for Zen meditation is Riding the Ox.
See Wikipedia article on Ten Oxherding Pictures.


  The calf, bull or ox is one of the earliest similes for meditation practice. It comes from the Maha Gopalaka Sutta (Majjhima Nikaya 33). It is also used in the commentaries, especially the one on the Maha Satipatthana Sutta (Digha Nikaya 22) and the Satipatthana Sutta (Majjhima Nikaya 10). As Buddhism spread throughout South-East Asia, the simile of the bull spread with it.


According to this explanation, the use of ox as symbol for meditation predates Zen, but on my memory I have not seen the ox used in any tradition other than Chan/Zen/Seon. Now technically, the ox is probably a symbol for the mind or something like that - so it is not the ox itself but riding it joyfully (symbolized by the flute) is what probably stands for the (successful) act of meditating.





Or perhaps, if you want to show the struggle you can use the fighting scene:



In Tibetan tradition the ox is replaced by an elephant:



In Pali Canon, the Buddha speaks on multiple occasions about herding the grazing cows, taming wild elephants, and training horses as metaphors for Buddhist practice. Chogyam Trungpa Rinpoche spoke about training our inner gorilla. Dza Kilung Rinpoche compared the gentle but steady approach to meditation with teaching a very young child. I think it should be safe to say that training an animal is probably a good symbol for meditation.

Other generic metaphors used by various Buddhist teachers for (not necessarily Zen, but definitely Buddhist) meditation, that come to mind, include: 


Improvising a certain theme on a lute, with the strings not tied too tense nor too loose.
Flying a kite of mind, tied to the stake of the breath.
Practicing airplane "touch-and-goes".
Watching the running water calm down.
Sobering up without sleeping. 
Cleaning the kitchen.
Weeding the yard or field.
Polishing a tile or a mirror.
Leveling jungle into a flat field.
Honing an arrow.
Heating or chilling a piece of smithwork to maintain the right temperature.


These metaphors hint at such factors as steadiness, balanced effort, removing the obstacles, and making incremental gradual progress.

Some of these might make good graphical symbols, too.