To change images use the file manager of the given installation (in this case: the one on server). You can choose to replace existing files or upload new ones. You _never_ work directly in the files directory to change any image on your site.

Hi thank you ... but if I make a change or add images from my local machine, how do I reflect this in the live site without messing with the database that will then have different link references I suppose ?

You want a new image on the live site? You go to the live site, login, get to the file manager and you upload it there. It's that simple. Like @mckoenig said, never ever mess directly with the files directory. Those folders are there to keep track of file versions and manage file sets.

Not sure If I am not very clear .... sorry if that's my english .. But what I am asking is how do I synchrnonize my work on my local machine with the site live ... I am not always working on the live site but first on my local computer then transfer the changes to the live site. The thing Is I 'm not sure how images are handled... so if for instance I add an image working from my computer, concrete will create one of those folders 4325 etc ... and reference the link to this folder in the database ... If I do that on the live site the system might create a folder with a different name or reference ...

Your english is fine, it was your requirements I wasn't getting!
You're thinking right, directories and files on the files folder are linked with database info, so you can't upload that folder to the live site. I usually do everything content-related on the live site, so usually don't have this problem.

Anyway, there is something that might work for you - in the files folder, you'll find a folder named "incoming". If you FTP your images directly to this incoming folder on your live server, you can then go to the file manager, click on "Upload Multiple" and click on the tab "Add Incoming" - the files you FTP'd will be there.

What I don't get is this - If you create a new page on the local server, the only way to put it on the live server is to upload the database. If that new page has an image, you'll have to upload the database AND the image. Why not do it directly on the live server? It's content!

Well you described exactly what my question was about !! I prefer to work on local first, mainly for speed issues, it is always much slower to work from the live site, and also because it is always good to work on the real thing with the right images etc . That is one thing, the other thing is that I think it is important that I have a complete backup in case of problems with the live server , so I can just upload the last image backup , files database and folder structure included. I really like the idea of having a single folder and subfolders for all images ... I find it a bit too much to have one image put in a folder and a subfolder .... I have over 200 images that I need to edit now to add copyright info to them and the "replace feature" would be just too long ....

I think you will just have to change the way you work. Either you do content work on the live site (where it belongs, normally) or you get accustomed to uploading your local files directory and updating your database on the live server every time you change something.
Best thing would be to work on the live site and when everything is done do a backup. That's how it is supposed to be.