Let's talk process flow first
You'll first need to transfer the image file manually to your web server (or for most of us this means to the web space we've purchased from our Internet Service Provider). This is usually done via FTP, using a program like WS_FTP.
This could also be done using a form as suggested previously, but you'd want to password protect that page so others couldn't replace your image, and it's not usually done this way.
Place the image in a separate "images" folder to keep it separate from your other html files.
Once the image is on your server, then it's just a matter of placing a link on your web page using the html in the previous reply.
Does that help?