>>2818957>>2818959There's two ways to rotate a JPG:
1. Adding an EXIF rotation tag
2. Actually modifying the image data itself
A lot of software (such as your phone's camera software, or the Windows image rotation function) will simply do the former, as it doesn't require rewriting the image data and avoids a small quality loss due to JPG re-compression.
However, EXIF data can also contain things like geographical coordinates, so 4chan intentionally strips all EXIF tags from images when uploaded (Except on /p/, the Photography board). This includes EXIF rotation tags.
Therefore, to rotate an image on 4chan, you have to actually modify the image data before uploading.
