This isn’t a Webflow bug. It’s a know issue with iPhone images.
This image was taken by an iPhone device: https://cl.ly/3j0J0p2d3a1w
Unfortunately, the iPhone camera hardware cannot save the image directly in rotated form. So rather than rotating the entire image, the camera software just adds a data file to the image noting which orientation the image should be in. It adds this information to the Exif data that all photos have.
So, when you open that photo with an application that reads the Exif data, the image is rotated while it’s not in other applications that don’t read the data.
So basically, this image is saved in its original, unrotated form, but the Exif data is making it show in the rotated actual position.
To fix this issue, you can open the image with an image editor tool such as Photoshop and save it again in the right orientation. Make sure to make it smaller in size also as this image is too large for the web. It’s recommended that you use images ~200-300kb or smaller, and background images can be 1080px or 1920px wide to make sure they run smoothly on your site.
Hope this is helpful.