\usepackage{xifthen}% provides \isempty test
\usepackage{xstring}

\newcommand\chapterillustration{}

\makeatletter
\setkomafont{chapter}{\let\@hangfrom\chapterhang\let\raggedsection\raggedleft\huge}
\makeatother
\newbox\chapternumberbox
\newcommand{\chapterhang}[2]{%
  \StrLen{\chapterillustration}[\imgnamelen]\ifthenelse{\imgnamelen > 1}{%
    \tikz[remember picture,overlay] \node[opacity=1.0,inner sep=0pt] at (current page.north){\includegraphics[width=\paperwidth,height=.59\paperheight]{\chapterillustration}};%
  }{}%
  \savebox\chapternumberbox{\fontsize{100}{100}\normalfont\sffamily\bfseries
    \thechapter}%
  \parbox[t]{\dimexpr\linewidth-\wd\chapternumberbox-1em}{%
    \raggedleft
    \makebox[1em][l]{\normalfont\normalsize\slshape\chapapp}\\
    #2%
  }\quad
  \raisebox{-1.75\baselineskip}{\usebox\chapternumberbox}\par
}

\iffalse
%https://komascript.de/comment/5067#comment-5067
\setkomafont{chapter}{\huge}
\makeatletter
% Hack: Eigenen, rudimentären Kapitelstil definieren.
\newcommand*{\scr@dsc@def@style@xchapter@command}[1]{%
  \scr@dsc@def@style@chapter@command{#1}%
  \expandafter\def\csname @@make#1head\endcsname##1{%
    \let\orig@raggedchapter\raggedchapter
    \let\raggedchapter\raggedleft
    \let\orig@hangfrom\@hangfrom
    \let\@hangfrom\chapterhang
    \scr@@makechapterhead{#1}{##1}%
    \let\@hangfrom\orig@saved@hangfrom
    \let\raggedchapter\orig@raggedchapter
  }%
}
\makeatother
\DeclareSectionCommand[style=xchapter]{chapter}
\newbox\chapternumberbox
\newcommand{\chapterhang}[2]{%
  \tikz[remember picture,overlay] \node[opacity=1.0,inner sep=0pt] at (current page.north){\includegraphics[width=\paperwidth,height=.59\paperheight]{\chapterillustration}};%
  \savebox\chapternumberbox{\fontsize{100}{100}\normalfont\sffamily\bfseries
    \thechapter}%
  \parbox[t]{\dimexpr\linewidth-\wd\chapternumberbox-1em}{%
    \raggedleft
    \makebox[1em][l]{\normalfont\normalsize\slshape\chapapp}\\
    #2%
  }\quad
  \raisebox{-1.75\baselineskip}{\usebox\chapternumberbox}\par
}
\fi

%\pagestyle{scrheadings}
%\ihead[]{\headmark}
%\ohead[]{\pagemark}
%%\cfoot[]{Selfhosting}
%\ifoot[]{ }
%\ofoot[]{ }