The handshape ILY is all-known in ASL culture. It's commonly used among ASL-speaking families and some close friends in Deaf community.
The three initials of I, L, and Y handshapes are blended into the ILY handshape with the thumb, index finger and pinky extended.
In LSQ (a signed language used in Quebec and eastern parts of Canada), this different handshape is a Canadian-French abbreviation of "Je vous aime" or "Je t'aime".
Many people mistook the ILY symbol for a horn symbol, overlooking or neglecting the significant difference of the thumb.
Cross-cultural story: In circa 1995, a hearing New York City entrepreneur gifted a (gold?) necklace with the ILY pendant to his wife who was a hearing Russian actor and coda (child of Deaf parents). When opening the gift, she was horrified by the "horn" symbol. No matter what he explained, the wife insisted it was the horn. Confused, the entrepreneur checked with me; I assured him that it was the ILY, not a horn or rock gesture (the fist with only the index and pinky fingers sticking out). What a semi-flop. :)
While it may appear to be a small difference to some hearing non-signing people, it's a huge difference between the ILY handshape and the horn handshape for Deaf/ASL speakers as much as (excuse this hyperbole) the difference between the sun and the moon (except for my then zero-year-old baby who thought the sun and the moon were the same thing, which was sweetly naive and forgivable). :)
Just in case not to confuse a naive hearing person further, the horn handshape (closed thumb) with the palm facing in toward one's body in ASL means bullshit. The palm orienation between palm facing in and facint out is as much distinct as the pronunciation between 'ball' and 'bull' or 'ball' and 'bill'. Except for some homonyms.
Every culture/language has its own set of meanings. One may interpret a single symbol differently from one culture or language to another.
The pedestrian stop light on the street in the photograph above surely makes an ASL speaker delightfully smile (and obviously it makes her/him stop and take a photograph) while another clueless person may frown at this broken light. Keep context and culture in mind.
Applause in Deaf culture.