Your comments

It's not only an iPhone issue, but also some applications do this as well, such as default Preview in Windows. Even PhotoNinja can make it happen if file is simply saved, not rendered into a new one. They all save orientation into EXIF.

The simple solution that I see is to add the ability to rotate the uploaded image. It might be the best solution of this issue, since it will also help with non-rotated images.

Here's my own example: I draw only in Autodesk Sketchbook on Galaxy Note phablet, but it never saves the orientation in which picture was drawn. Then if I render it as jpg (for a complex scene), Android will have an option to rotate the image from Gallery, but if the scene is simple, it's better to use png instead, but the problem is that Android can't rotate png, so I have to save it in jpg or send png to PC and rotate it there (which isn't comfortable at all).