Q:

Figure is oversized horizontally in tcolorbox

I insert a figure into tcbitemize environment of tcolorbox.
The figure width is set to \textwidth. So the figure should be fully in box.
In fact however, I get a horizontally oversized figure, that is to say, the right side of the figure is out of the box.
Why and how to solve it?
Here is the MWE:
\documentclass{article}
\usepackage[most]{tcolorbox}

    \begin{document}

    \tcbset{clearsep/.style={%
        boxsep=0pt,left=0pt,right=0pt,top=0pt,bottom=0pt}%
    }

    \begin{tcolorbox}[clearsep,colframe=orange,boxrule=2pt,sharp corners]
      \begin{tcbitemize}[clearsep,boxrule=2pt,colframe=green,sharp corners,raster columns=1]
         \tcbitem \includegraphics[width=1.0\textwidth]{logo_st}
      \end{tcbitemize}
    \end{tcolorbox}

    \end{document}[!

A:

The logp source image￼ is too cropped on the right (even is lost a  bit of the semicircular shape). Also the space at top and bottom is less that in the left side.
Some solutions to correct this are:

Fix the source image with a paint program as Gimp.
Reduce a bit the width of impressión the image, to .92\textwidth for instance,  so at least the logo will appear centered in the box, but that will leave more space in the sides that in top and bottom.

Use the trim option of \includegraphics to correct the spaces at both sides (change the 6 and -5 values of the MWE as needed):

\documentclass{article}
\usepackage[most]{tcolorbox}
\begin{document}
\tcbset{clearsep/.style={boxsep=0pt,left=0pt,right=0pt,top=0pt,bottom=0pt}}
\begin{tcolorbox}[clearsep,colframe=orange,boxrule=2pt,sharp corners]
\begin{tcbitemize}[clearsep,boxrule=2pt,colframe=green,sharp
 corners,raster columns=1]
\tcbitem \includegraphics[width=\textwidth,trim=6 0 -5 0]{NqH4C.png} 
% Note: NqH4C.png is the second image from the question *as is*, 
% but *without any space* in the filename. Just download it 
% without renaming will not work !!
\end{tcbitemize}
\end{tcolorbox}
\end{document}

Anyway, it is always a bad idea to enlarge too much an PNG or JPG image. Any image of bitmap type should have a final resolution of at least 300 dpi (dot per inch) to be printed with a decent quality. Your image have only 148 pixels, so it should not be printed ideally with a width of much more than 1.25 cm to look smoor to the human eye. 
Instead, vectorial images, as SVG converted to PDF, or a tikz figure could be enlarged as you want without any loss of apparent quality. 
Edit: 
The OP is still convinced that the problem are not only the images and add a mushrooms image to demostrate the problem, but just changing the logo by this image in the MWE of the question, the image fit perfectly in the green frame, even zoomed at 1600% in a xpdf viewer. The following capture of the bottom right corner is made only with a zoom of  400% to allow to check that it was made with the mushrooms image:
 
Therefore, maybe the OP have a problem of an outdated TeX installation, or a PDF viewer with a bad zoom, but  anyway  seems a  "too localized" problem to provide any solution. 

