I have it too.
As does DP.
I took a picture of our chilli plant to show that the first chilli had gone red.
A friend sent me a picture of her new nail varnish.
You only use it to message people whose phone numbers you have (who he could already have been sending naked pictures to if he wanted).
Maybe someone said to him "hey, try this app", and he downloaded it, used it once, and decided that he wasn't interested.
Unless you have other reasons to suspect that he is up to something, simply owning the app doesn't indicate anything.