No, this is the fundamental disconnect that's broken this conversation at least twice in the thread, so far as I can tell.
There is no "technical" way that the Wonder Woman costume is a spoiler. The hypothetical "casual observer" we're allowing for is a skewed hypothetical closer to a straw man than anything. There's no solid definition for "casual observer" and you can rewrite how "casual" you want this observer to be as a means to fit whatever argument you're trying to make, usually in an attempt to show people your perspective isn't out of whack regarding the hurt you feel at being "spoiled."
But Wonder Woman's costume reveal is not a spoiler. Gal Gadot was announced as playing the character long ago. To the point where her contract negotations became news in and of themselves afterwards. The choice by the movie studio and the director of the film to release the photograph of who she is and what she looks like to the audience dictates that the information, both textual and visual, is not a key plot component that will negatively affect the storytelling of the film or the enjoyment of the story.
It's not a spoiler. Nothing's being spoiled. Who the character is, Who is playing that character, and what that character looks like isn't a spoiler. That's a poster. That's a trailer. That's the credits. That's pre-release marketing and audience education.
It's disingenuous to take something as basic and straightforward as that, and then use the nebulous "casual observer" to manipulate that into some level of aggressive inconsideration on the part of people who are taking freely disseminated information BY THE STUDIO and wishing to have a conversation about it.
Again - if I choose to maintain a level of "purity" regarding my first viewing that cuts out even the most basic of information, then that's a "me" thing, not a "rest of the world" thing. I have to have perspective enough to realize that I'm going above and beyond. I'm not being "casual" about it, and I need to decide how best to spend my time online in order to maintain that "purity."