% inputpath.sty
\RequirePackage{etoolbox}

\def\inputpath@pgf@findfile#1:#2+#3{%
  \ifx\Ginput@path\@empty
    % There's nothing we can loop over: pass the arguments to the original
    % command as is.
    \inputpath@old@pgf@findfile{#1}:{#2}+{#3}%
  \else
    \let\inputpath@cur\@secondoftwo
    \expandafter\@tfor\expandafter
      \inputpath@cur\expandafter:\expandafter=\Ginput@path\do{%
      \def\inputpath@filename{\inputpath@cur/#3#1}%
      \pgfutil@IfFileExists{\inputpath@filename}%
      {\xdef\pgf@filename{\inputpath@filename}%
        % Found the file. We're done.
        \@break@tfor
      }%
      {\def\pgf@mightbeempty{#2}%
        \ifx\pgf@mightbeempty\pgfutil@empty\else%
          \inputpath@pgf@findfile#2+{#3}%
        \fi
      }%
    }%
  \fi
}%

\newcommand\inputpath@pgf@declareimage@fail{%
  \PackageWarning{inputpath}{\string\pgf@declareimage\space could not be
    patched\MessageBreak
    to enable graphicx search path support.\MessageBreak
    Using \string\graphicspath\space may not extend the\MessageBreak
    image search path\@gobble}%
}

\AtEndPreamble{%
  \ifx\pgf@findfile\@undefined\else
    % Add search path support to \pgfdeclareimage
    \let\inputpath@old@pgf@findfile=\pgf@findfile
    % Uncomment the following line for debugging purposes:
    % \tracingpatches
    \PackageInfo{inputpath}%
      {Patching two occurrences of \string\pgf@findfile\space in\MessageBreak
      \string\pgf@declareimage\space for enabling\MessageBreak
      graphicx search path support\@gobble}%
    % As of TikZ/PGF 3.1.8b, \pgf@declareimage contains two calls to
    % \pgf@findfile. Both references to the original macro must be replaced.
    % Check pgfcoreimage.code.tex if patching succeeds but \pgfdeclareimage
    % still cannot locate images in the graphicx search path.
    \patchcmd\pgf@declareimage
      {\pgf@findfile}%
      {\inputpath@pgf@findfile}%
      {%
        \PackageInfo{inputpath}{Patched first occurrence of
          \string\pgf@findfile\space in\MessageBreak
          \string\pgf@declareimage\@gobble}%
        \PackageInfo{inputpath}{Patching second occurrence of
          \string\pgf@findfile\space in\MessageBreak
          \string\pgf@declareimage\@gobble}%
        \patchcmd\pgf@declareimage
          {\pgf@findfile}%
          {\inputpath@pgf@findfile}%
          {\PackageInfo{inputpath}{Successfully patched
              \string\pgf@declareimage.\MessageBreak
               It is now possible to use \string\graphicspath\space\MessageBreak
               to extend the image search path\@gobble}}%
          {\inputpath@pgf@declareimage@fail}%
      }%
      {\inputpath@pgf@declareimage@fail}%
  \fi
}

\graphicspath{{./images/ch00/}}