This is where you need to know about website development and how this whole web site thing works

There isn't really a good answer I can give you because there are too many possibilities. At the most basic level, you might need to adust some html code to refer to your new image file, but you might have a CMS (content management system) which requires changes to a configuration to include the new image instead. Or something else.
All in all, if you have a web site which is important to you and you don't have any knowledge at all regarding web site development, then I strongly suggest hiring someone who does.
(by the way, I answered your email but your ISP rejected my reply... no idea why)
