You have an iphone? this happened to me and I had to d/l them to my desktop, rotate them four times in preview, save them and then it was all AOK. #lifehack
Here's another weird thing, if i look at this page (the web page) in the browser on my phone (Safari or Chrome), the photo is correct. If it look at it with a desktop browser (Chrome or FF), it's wrong.
tl;dr: rotate the photo in Photos first, then post.