You say that you don't want downloadable software, but CMSs usually don't require the Client to download the software, just the web page designer. The Client just has to log in to be able to edit the text or images.
Snippetedit is regularly mentioned on this board, it is very simple, some people have said that it is too simple, but it is designed for people with no website coding knowledge.
http://www.snippetedit.com/
I have tried the demo and it seems simple enough. Editing images needs care; the page must be coded so that it fixes the size (not part of the code that can be edited) so that if different images are uploaded the browser resizes them. There are no design features, you would have to get a template from somewhere else and build in the snippetedit codes.
Joomla is a much more detailed CMS and some training might be required for the Client to use it.
http://www.joomla.org/
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.