... Is there a way to preview it and correct that when I'm posting?
It would help to view it in a separate image viewer app. And then, you can rotate it within that. If you view an image in the same device it was taken with (smartphone or other digital camera), that setup often "helpfully" makes images appear oriented correctly when they aren't.
A common example is of a smartphone locked into portrait mode when the image was taken, but it can happen other ways. I transfer to, and then post images from my computer because of the flexibility that offers.