|If you are replacing your picture, subject to the limitations of being a jpg file and the file size limits, if you encountered no errors then your picture will have been updated.
.. But it's still showing the old one ...
This is because the image has the same name, your computer is showing you what it knows is there, and not what is actually there.
Just refresh the page to force your computer to reload the page. Hold down your Ctrl key and click on refresh, or hold down the Ctrl key and hit the F5 key.
This should now force your computer to show you the most recent version.