Some pictures appear rotated when you think they should not. There has been a long discussion raging for many years on the subject and has become worse with the advent of smart phone camera images. The quote below explains in detail but as an example look at the attached images which is a set of the 8 EXIF orientations for a portrait and a landscape image. Look how the thumbnails are displayed and then how the full size image is displayed in whatever browser you are using by clicking on the thumbnail.
There is also discussion raging in the phpbb forums on the subject, mainly centred on whether phpbb should know better than the person who uploaded the image or should store and display as provided by the user. I am looking at a facility to allow users to orientate(rotate) the images similar to that available in the old JowettGallery. I am extremely wary of adding extensions to the supported software to accomplish this. Note that when the images in the JowettGallery were moved the original and not the latest version of the image (such as one that had been rotated) was uploaded as an attachment.
The author mixes multiple issues - if you trace it down to what happens where then you understand more clearly the issues:
When picture files are created, their dimensions are always correct. There is not one single picture file where one needs to think "is width actually the height?" - such thinking only happens by omitting other details.
When the device which created the picture is able to provide more details (camera was held rotated) then it may store this as EXIF metadata. Let's all assume the device does this in the correct way.
EXIF metadata: is it present or not? When having a file without EXIF metadata nobody knows if it was recorded rotated - neither your internet browser, nor any other software.
If a software can interpret EXIF data then it is also free to display the picture in a different way (without modifying the file). Rotated 90° clockwise? Sure, let me display it in such a way. The original file dimensions are still 1920x1080, it's just that what is displayed is now presented in 1080x1920 due to rotation. Internet browsers try to do this, and picture viewer programs in general.
If a software can interpret EXIF data then it may modify the file. Rotated 90° clockwise? Sure, let me transpose the data so the file dimensions become 1920x1080. I should eliminate that part of EXIF metadata which initially led me to do this, tho (a couple of software does not so, and then other software still interpretes the EXIF metadata and acts accordingly). ImageMagick and a couple of websites do this.
(Monitors can be rotated, too. Most graphic card drivers are able to rotate their output likewise. No software is expected to recognize your monitor's orientation, and luckily no software does so - it's up to the driver/operating system to control this.)
That's the whole magic. And it has to be understood in a cascading way:
If the internet browser interprets EXIF metadata to display a picture differently then no other software (i.e. the website) should interfere. Problems then apply to internet browser bugs. This also means: don't upload your files to websites/online storages which modify your files - if you do so then you are part of the problem.
If the website wants to modify the picture files according to their EXIF metadata then it must kill anything that could be (re)interpreted by the internet browser. Problems then apply to the website. This also means: those are no original files anymore - if you're not doing backups in case something goes wrong then you are part of the problem.
If the device stores wrong/no EXIF metadata to begin with then problems apply to that one. This also means: if you don't check that yourself or you do/can not update the firmware then you are part of the problem.