Depends on how the site is structured and coded...
Basically you would login via FTP or use the cPanel file manager and download the page file you want to work on. Make the changes, then upload the modified file and the new images up to the server.
If you want to provide more info about how the site is setup - that would help us to help you.
Thanks for the reply.
Not sure what you mean by "structured and coded". I can click on the page file in the public/html directory and edit it using the html editor. But I can't move the images about on the same page.
@WebleyBe careful using that type of editor because it would be very easy to break the site. The built in HTML editor in cPanel is extremely basic and does not understand the underlying structure of the website. If you were to click the Source button you can see the actual code that makes up the website, the editor is simply trying to interpret this similar to how a web browser would.
Generally I recommend working on the website offline and only updating the hosted files when changes are ready to go live. You can use an FTP client like Filezilla to download and upload the files, then on your own computer you can make a copy of the folder each time you're going to make changes so you'll have something to fall back to if it doesn't go as planned.
But the code in those files makes up the structure that @Muse mentioned. There's 'hidden' information that tells browsers that this image goes there, but many different ways that can be done. It is all up to how it's coded so it's hard to say without seeing it. If you can provide the URL to the site and a brief description of the change you're trying to make.
What @Nate said!
Moving images around means changing/modifying code to get them to display the way you like. If you are going to do that, make a copy of that file, so that if you do not like the results, you can always restore back to the original.