The story goes like this.. the letter of the word fish in greek stand for some acronym that stand for something christian. My greek is non-existent so I can't remember. I think it comes down to Jesus Christ somehow
I believe you're direction above is correct if it indeed matters..
But then signs is dangerous business. The way I understand the bible we shouldn't be bothered with them. I don't even where a cross, not that I have something against other people wearing it

I am against crucifixes though.
Most signs you see around have more than one meaning. And mostly they either have pagan or witchcraft connetations.
the sign above has some ancient pre-christ pagan connection. I forget what sorry. Do you want me to go search..
Some other examples:
The 5 pointed star pointing up is used in new-age witchcraft secular huminism and some other.
The 5 pointed star pointing down is used in Satanism. Many times with the goats head drawn inside. and a circle around.
The cross with a circle around (with the one leg sticking out) many times known as the celtic cross is basicly a witchcraft circle of power with just one leg extended.
On some bibles and church song books they print signs that is actually free masonry signs. I know there is this P and X into each other thing I've seen on some churches things.
And most free masonry signs and stuff can be traced back to Satanism in one or other form.
The sign of greeting with 2 fingers and the thumb inside (which I've actually seen in paintings from the early church) is also something in freemasonry but the origin can be traced to some pagan priestly stuff before christ.
The eggs and bunnys on Easter is fertility signs.. Since the date we use for easter is the feritlity festival date of the pre-christ pagan europe. And is still today the high of the pagan calender.
The christmas tree is pagan. Infact the bible is very clear about christmas trees: Go read Jeremiah 10
Misteltoe is pagan (also some fertility ritual)
And the date we use for christmas is also the date of some great pagan festival in the per-christ europe world.
These things get really dark and nasty so I'm going to stop now.
After all paul says we must rather focus on the light than the dark.
There is complete books on this stuff. hehe.
Well don't let it bother you.
Put on your poster what ever you want. The bottom line is that any "sign" has multiple interpretations and was probably used more than once in history. You're fish sign today is mostly considered connected with christianity in some way. So I would say it is perfectly ok to use it.
The most important thing is that we do not get sidetracked and always keep our eyes on Jesus.