I see some idols have it mentioned on their pages, but there is actually no such thing as image stat specialty in deresute.
Each card has it's own skill and stat affinity.
Kaede, for example, has two SR cards, one of them is Visual and the other one is Dance; one R and one SSR cards, wich both are Vocal.
It's the same for most idols with several cards. Even if it's not the case right now, more cards are to be added in the future, so it will definetely be.