You can use
joomlaxplorer, a component you can find here:
http://extensions.joomla.org
. You will be able to edit, delete, copy, rename, upload files, and change file permissions if necessary.
Or, if you have a
FTP program, you can also replace your images, but sometimes, you have not the required file permissions. This is where joomlaxplorer is very useful.
If it's still not clear, don't hesitate to ask more info.