How was your website written in the first place?
If you understand website coding (HTML et al) - then you can do anything you want with a text editor.
You can download Komposer for free - whether they will help you depends on your existing website and what you understand about website construction.
No - you can't "copy and paste" over an old image in the way I think you mean. You could upload a new image with the same name and properties as an existing one, if you know how to ftp to it.
Komposer click here