Generally, you can simply upload an image and it will appear correctly online. Occasionally, however, especially with pictures in portrait mode from iPhone's, the image will appear on it's side.
The easiest way to fix this is to open the image and rotate it then save the changes.
When you rotate an image 360-degrees that appears correctly on your computer or phone it removes data that confuses the browser.
Here is an example:
After rotating and saving, reupload the image and it should appear correctly.