Raised middle finger to the sky.

I thought it was this one, the Flying Spaghetti Monster:

Given that USA has become the leading genocidal force from Florida to Juneau and Honolulu in 236 years and expanded the xian zionist assassins invading over 150 countries since 1946, the flag and slave song of 1812 drowns out the American Atheist Movement...... symbols matter sadly more than human life to the violent theocrats globally
....we Atheists are growing over 20% of USA citizens and we can reverse the global religious violence when we start voting for secular CANDIDATES to become incumbent new symbol bearers for peace

The official motto of USA is E PLURIBUS UNUM and zero coins or currency used the gawd symbol until the 1830's VERY UNPOPULAR with secular citizens and instantly repealed from minting 1864 theocrats pushed for a 2 cent coin and that also failed for UNPOPULAR useage....theocrat Teddy Roosevelt demanded the 1909 coins bear the stamp IN gawd WE TRUST....1955 the same liar symbol was printed onto currency.....American Atheists and the great Infidel Agnostic Ingersoll won many Blaine Amendments requiring secular taxation and policies....Bradlaugh in Parliament 5 times refused to "swear gawd & Queen" ....the secular freethought Atheist movement is 27 centuries old with Thales and Materialism..... sanitizing and falsely defining Atheism and Atheists history is the business of theocrats none of us should fall into their dishonest traps

Objectivist founder Ayn Rand declared the symbol for her alleged "philosophy" is the " $ " dollar sign pissing on pounds francs kruegerands marks lire pesos dinars et al worshipping "atheistic" naked International GREED.... PEIKOFF argued it was USA duty to kill anyone who got in the way of oil profiteering during the USA invasion of Kuwait Saudi Arabia and Iraq....planting many thousands of xian crucifixes thereby

I’ll make this more prominent: “The atomic whirl is the logo of the American Atheists, and has come to be used as a symbol of atheism in general as some American Atheist members claim.” [atheists.org]

There's been a bunch put out there, but I don't think one particular one has been designated.
[conservapedia.com]

some associations have a symbol, but they never can grow too much or even stay too much time alive.
It is difficult to gather people with different world views and backgrounds together without a false cosmic nany that dictates this.

It is NOT drawing a fish in the sand with a big stick. People that so that want to get to Pieces and are making up some Jesus myth.