Well you actually make the changes to a copy of the page on your local computer, then upload the new copy to overwrite the old one.

An easy trick for finding the code is, when you view the page in your browser, right click on the image and select properties, there you will find the file name. Copy it, and open that page on your computer in notepad, ctrl-F is a page find. Paste the file name in, press enter, and it takes you right to it, highlighted. You can then even use notepad's replace to put the new one in it's place.