post image with no padding

With a little bit of css is not difficult, you can see the results here:
http://gpportfolio.artomultiplo.net/blog/

I just added this code in Simple Css plugin

 .single-post .inside-article, .blog .inside-article{
	padding-top: 0px;
}

.post-image, .page-header-image-single {
	margin-left: -40px;
	margin-right: -40px;
}

You should adjust the values (-40px) in relation with your "Spacing" values. It should work with and without sidebars.

And that's it :-)

Thank you so much with your help
actually I want it to appear like this one -> organicbook.com

The text on the picture is done directly in the image editor not in WordPress. The CSS in the post let you achieve what you need.
You should remove the excerptand add some CSS to the read more.

The only thing I don't know exactly how to do is to display the taxonomy on top left.

Adding the taxonomy like that would require some custom programming for sure.