<hand waving>
You can colour a pixel according to the colour of a pixel in an image included with \includegraphics
. At least, you can do so indirectly. It does not, however, follow that PGF/TikZ has access to information about the colours in the images. I suspect it does not. Rather, I would guess that it tells the backend driver how to colour the pixel based on the colour of the pixel in the image. Or something like that ...
</hand waving>
For example, here are four renderings of the standard tiger
image. These were rendered using pdfTeX and viewed in Okular on GNU/Linux. They may well look different if the code is compiled with other engines and/or viewed in other PDF viewers and/or on other systems.
\documentclass[border=10pt]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\begin{scope}[blend mode=difference]
\node {\includegraphics{tiger}};
\shade [bottom color=blue, top color=green, middle color=magenta] (current bounding box.north west) rectangle (current bounding box.south east);
\end{scope}
\end{tikzpicture}
\begin{tikzpicture}
\begin{scope}[blend mode=screen]
\node {\includegraphics{tiger}};
\shade [bottom color=blue, top color=green, middle color=magenta] (current bounding box.north west) rectangle (current bounding box.south east);
\end{scope}
\end{tikzpicture}
\begin{tikzpicture}
\begin{scope}[blend mode=exclusion]
\node {\includegraphics{tiger}};
\shade [bottom color=blue, top color=green, middle color=magenta] (current bounding box.north west) rectangle (current bounding box.south east);
\end{scope}
\end{tikzpicture}
\begin{tikzpicture}
\begin{scope}[blend mode=color dodge]
\node {\includegraphics{tiger}};
\shade [bottom color=blue, top color=green, middle color=magenta] (current bounding box.north west) rectangle (current bounding box.south east);
\end{scope}
\end{tikzpicture}
\end{document}

\includegraphics
is relying on the backend driver (engine dependent). You might be able to use PGF's own image-inclusion stuff. This is not recommended (by TikZ/PGF) unless you really need it, but there are some things it can do whichgraphicx
does not support.lualatex
but not something I'd try doing. Perhaps easier with pythontex.