The original picture will display correctly when you open it on your computer. The problem is the photo viewer.
Most photo viewers are designed to automatically correct the image you're looking at. If the photo was meant to be displayed in portrait layout, that's what you will see.
This is a nice feature, but also a very deceiving one. When you upload your photo, you're actually uploading the true file - so once it goes live it will display incorrectly.
The fix: open your photo editor, rotate the image 90 degrees, then another 90 degrees so it’s back to normal. Save the file and re-upload. By doing this, you're forcing the original file to be corrected.