If you're a Lemmy dev and reading this, the problem is in pict-rs. I have sent an email to asonix with the needed changes, please tell them to check their inbox (since I can't register on their git server, I can't submit a formal PR).
Send me a PM if the email gets lost and I'll give you the line you need.
If you're not a Lemmy dev:
Have you encountered an image that is suspiciously rotated here on Lemmy? Perhaps you even tried posting an image that looks right yourself and found it rotated itself! Why?!
The reason is that Lemmy strips all metadata from images you upload to it. This is because image metadata can contain, among other things, GPS coordinates or where it was taken. The problem is that when you take a picture with your phone in landscape, instead of rotating the image in memory, your phone saves the image sideways (because that's how it came off the sensor) and then adds a metadata tag that tells everyone to rotate the image as they are displaying it. You guessed it, that tag also gets deleted. In most cases, this is fine because either the picture wasn't rotated to begin with, or Lemmy image hosts actually save the properly rotated image before stripping the tag, but in some image formats, this isn't the case due to a programming oversight. I have found the fix and sent it to the person responsible for the image hosting code.