Ah ha! A big-ass Featured Image will come through as a preview image. Me trying to use smaller images, apparently, screwed me.
Okay, as best I can tell, MastoWeb uses OpenGraph information to construct their preview cards, which does include the post's Featured Image (as OpenGraph thumbnail) as long as it's big (1920x1920 works, 300x300 does not).
Ivory, by contrast, either sources OpenGraph and then doesn't use half of it or uses Twitter Card data. (It doesn't display website name.) It, too, wants a BIG Thumbnail or else it will post no preview card whatsoever.
@lisamelton The "big" threshold for MastoWeb seems to be pixels and not file size. I have a post with a 1000x563 jpg (710kb) thumbnail that did not get included, and a post with a 1000x772 (228kb) thumbnail that did. So the threshold is somewhere between those two sizes, I guess?
@miriamrobern That's one Hell of a heuristic! 😂
@lisamelton Workin with the data I've got! ;)
@miriamrobern Well, that's what ya gotta do amidst such madness. 😂
@lisamelton Yeah, confirmed: I sized up the Featured Image on a post from Monday that didn't get a preview, and now it does have a preview.
@miriamrobern That is soooo... weird and almost arbitrary. 🤦♀️
@lisamelton IME everything about preview cards is totally arbitrary.