This is probably do to the EXIF/TIFF metatags that are in the images. One of these tags indicates the orientation of the image. These tags are added to image by the device used to take the picture and come from the settings you have on that device.

Computers and browsers interpret the camera metadata (details about image, including portrait/landscape) differently which is why you may notice a difference when viewing it in your computer's file explorer, email, phone..etc
Also some browsers read this orientation info and rotate the image accordingly, others do not.

Some photos you upload to a server (eg from the iPhone) may have these tags, others (from a different source ) may not. The server may or may not retain these tags, and may or may not read the tags in order to attempt correct orientation of the image. Similarly a web browser may or may not pay attention to these tags. If you upload images containing tags the results are unpredictable

When you come across an image that is displaying sideways it's best to take that image into a graphics program, rotate it ( if it's sideways in the program ) and then save the image. That should remove the tags from the image so it displays correctly.

You can use any graphics program you may already have on your computer or something like:

Paint.NET

Or an online editor

https://pixlr.com/editor/

http://www.fotor.com/