\newcounter{themenumber}
\newcounter{classnumber}
\newcounter{imagenumber}
\forloop{themenumber}{1}{\value{themenumber} < 24}{
    % \hline <-- Error here
    \arabic{themenumber}
    \forloop{classnumber}{0}{\value{classnumber} < 2}{
        \forloop{imagenumber}{1}{\value{imagenumber} < 6}{
            & \includegraphics[scale=2]{
                ../../results/\arabic{themenumber}/\arabic{classnumber}_\arabic{imagenumber}.eps
            }
        }
    }
    \\
    \hline
}