Thank you very much for you help.
I now have a smaller tumbnail on my post archive page as I wanted but there is some hitch with the frame, as you can see here: [IMG]http://i46.tinypic.com/34goojd.png[/IMG]
Do you have any idea to solve it?
I have searched the web with no result. I confirm that the image was re-uploaded to avoid any update issue.
Thank you for any clue.