\documentclass[
  year = 2018,
  imageFolder = ./photos/,
]{wallcalendar}

\makeatletter

\usepackage{fontspec}
\defaultfontfeatures{Ligatures={TeX}}
\setmainfont{TeX Gyre Pagella}
\newfontfamily\dejaVuSans{DejaVu Sans}

\newlength\@wall@tmp@a
\newlength\@wall@tmp@b

\newlength\@wall@thumbWidth
\newlength\@wall@thumbHeight
\newlength\@wall@thumbCaptionWidth
\setlength{\@wall@thumbWidth}{0.1749\calPaperWidth}% 30mm at the 6.75in page width, 0.1749 = 1/5.715
\setlength{\@wall@thumbHeight}{\@wall@thumbWidth}
\setlength{\@wall@thumbCaptionWidth}{0.2333\calPaperWidth}% 40mm at 6.75in page width

\newcommand\thumbFmt{}
\newcommand\thumbMonthFmt{\fontsize{10}{13}\selectfont\bfseries}
\newcommand\thumbCaptionFmt{\fontsize{10}{13}\selectfont}

\def\@wall@thumbFile{}
\def\@wall@photoCaption{}

\newcommand\ThumbWithCaptionLeftSide[1]{%
\pgfkeys{/Photo/#1/thumbFile/.get=\@wall@thumbFile}%
\ifx\@wall@thumbFile\empty
  \pgfkeys{/Photo/#1/file/.get=\@wall@thumbFile}%
\fi
\pgfkeys{/Photo/#1/caption/.get=\@wall@photoCaption}%
% Thumbnail caption
\ifvarnishmask%
\hspace*{\@wall@thumbWidth}
\else%
\begin{minipage}[b][\@wall@thumbHeight][t]{\@wall@thumbCaptionWidth}%
\raggedleft
\thumbFmt
{\thumbMonthFmt \@tr@monthNumName{\monthToNum{#1}}}\par
{\thumbCaptionFmt \@wall@photoCaption}%
\end{minipage}%
\fi%
\hspace*{3mm}
% Thumbnail photo
\begin{minipage}[b][\@wall@thumbHeight]{\@wall@thumbWidth}%
% FIXME placeholder
%\placeholder{%
\includegraphics[ keepaspectratio, height=\@wall@thumbHeight ]{\@wall@thumbFile}%
%}%
\end{minipage}%
}

\newcommand\ThumbWithCaptionRightSide[1]{%
\pgfkeys{/Photo/#1/thumbFile/.get=\@wall@thumbFile}%
\ifx\@wall@thumbFile\empty
  \pgfkeys{/Photo/#1/file/.get=\@wall@thumbFile}%
\fi
\pgfkeys{/Photo/#1/caption/.get=\@wall@photoCaption}%
% Thumbnail photo
\begin{minipage}[b][\@wall@thumbHeight]{\@wall@thumbWidth}%
% FIXME placeholder
%\placeholder{%
\includegraphics[ keepaspectratio, height=\@wall@thumbHeight ]{\@wall@thumbFile}%
%}%
\end{minipage}%
\hspace*{3mm}
% Thumbnail caption
\ifvarnishmask%
\hspace*{\@wall@thumbWidth}
\else%
\begin{minipage}[b][\@wall@thumbHeight][t]{\@wall@thumbCaptionWidth}%
\raggedright
\thumbFmt
{\thumbMonthFmt \@tr@monthNumName{\monthToNum{#1}}}\par
{\thumbCaptionFmt \@wall@photoCaption}%
\end{minipage}%
\fi%
}

\newlength{\thumbColumnWidth}
\newlength{\thumbColumnHeight}
\newlength{\thumbSep}
\newlength{\@t@a}
\newlength{\@t@b}

% vertical spacing
\setlength{\@t@a}{0.2\textheight}

% horizontal spacing
\setlength{\@t@b}{0.2\@wall@thumbCaptionWidth}% 8mm at 40mm caption width

\ifdimcomp{\@t@a}{<}{\@t@b}{
  \setlength{\thumbSep}{\@t@a}
}{
  \setlength{\thumbSep}{\@t@b}
}

\setlength{\thumbColumnWidth}{0.5\calPaperWidth - \thumbSep}
\setlength{\thumbColumnHeight}{\textheight}

\newcommand\ThumbsPage{%
\clearpage

\hspace*{-\@wall@leftMargin}%
\hspace*{-1pt}% small correction, space gets in somewhere
% Wrap
\begin{minipage}[t][\thumbColumnHeight]{\calPaperWidth}%
\centering%
\setlength{\parindent}{0pt}%
\setlength{\parskip}{0pt}%

% NOTE: multicols will not \vfill
% Left Column
\begin{minipage}[t][\thumbColumnHeight]{\thumbColumnWidth}%
\raggedleft

\ThumbWithCaptionLeftSide{January}

\vspace*{\thumbSep}

\ThumbWithCaptionLeftSide{March}

\vspace*{\thumbSep}

\ThumbWithCaptionLeftSide{May}

\vspace*{\thumbSep}

\ThumbWithCaptionLeftSide{July}

\vspace*{\thumbSep}

\ThumbWithCaptionLeftSide{September}

\vspace*{\thumbSep}

\ThumbWithCaptionLeftSide{November}

% End of left column
\end{minipage}%
%
\hspace*{\thumbSep}%
%
% Right column
\begin{minipage}[t][\thumbColumnHeight]{\thumbColumnWidth}%
\raggedright

\ThumbWithCaptionRightSide{February}

\vspace*{\thumbSep}

\ThumbWithCaptionRightSide{April}

\vspace*{\thumbSep}

\ThumbWithCaptionRightSide{June}

\vspace*{\thumbSep}

\ThumbWithCaptionRightSide{August}

\vspace*{\thumbSep}

\ThumbWithCaptionRightSide{October}

\vspace*{\thumbSep}

\ThumbWithCaptionRightSide{December}

% End of right column
\end{minipage}%

% End of wrap
\end{minipage}%
}

\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{January}
\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{February}
\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{March}
\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{April}
\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{May}
\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{June}
\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{July}
\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{August}
\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{September}
\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{October}
\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{November}
\SetPhoto[file={frog-crop}, thumbFile={frog-crop-thumb}, caption={Let the frog jump}]{December}

\makeatother

\begin{document}

\ThumbsPage

\end{document}