My guess is that the forum server-side software is not respecting the EXIF metadata about image orientation when it generates the thumbnail, leading to a thumbnail that is rotated compared to the original image.

You can also see this when you rotate and save an image in software that doesn't update the EXIF data correctly, leading to an image that looks right sometimes but then gets rotated to match the EXIF data when viewed in an EXIF-compliant viewer.

All very frustrating and technical, but that's what I think is going on.

According to this, some of the built-in Windows image tools don't handle EXIF data correctly. I am not sure which ones do and which don't though.