Hi soufatn, I know of a 2.6.1 site that runs the plugin fine, so I wouldn’t draw that conclusion. Unfortunately, the diagnostic page is not able to identify all problems. For example, I’ve had some users tell me that the GD Library wasn’t properly compiled (and that it shows as being installed) and upon re-compiling, it worked.
Are you installing the plugin anew? Or did you upgrade from a previous installation (if so, from which versions of WordPress and the plugin)? What is your site’s URL? What error messages, if any, do you get upon page load or when loading the image directly?