What is happening is that the phone includes something called EXIF data. That is meta data with info about the picture. It is usually the time the picture was taken. It can also be information of what camera (or phone) was being used, the location (if this feature is turned on in the phone) etc. But one of the most important things is orientation. The sensor is always at the same orientation, so the EXIF data will tell which way is correct. This works 99% of the time when you take usual pictures as the camera detects the gravitation orientation. But when you take pictures of papers you hold your phone dead flat. Then the orientation is hard to detect, and it usually goes for the last orientation it had before you oriented it to the plane, or it might select the default.
On my phone I can edit each picture afterwards and then change the orientation. But another trick is to remove the EXIF data. I did that on your other pictures last night with my computer.
When you upload the pictures to the forum, the forum makes a thumbnail version of the photo. When that happens it does NOT include the original EXIF data with orientation information. You therefore end up with a thumbnail with a different orientation than the original. I have been bitten by this many times as some software reads this information and reorients, and others don't.