After a long search, I find your service to be the only one that is doing what I want, I have been using firefox screengrab! extension, but being an opera user, I don't really want the need to have firefox installed, and run it just for that.
However, I do not feel your service is still a good replacement, for the simple fact that the output image format is in .jpg, a crappy quality .jpg image, wich for me (and others I know of) is not really an option, as a good quality picture is needed, so I would recommend an option (or even the default) to have the images in .png format, because quality can always be reduced if needed, but it cannot really be enhanced when it is that bad.