So here is my take. The TOC caption optional argument is probably not ideal where it is now but should instead come before or after the cation argument. Also caption and label could be optional arguments, too. And then the code could be considerably cleaner I guess. The distinctions of cases blow this code up more than should be necessary but I was unable to use the same approach for landscape and portrait somehow.
% Provides full page picture command
% \picturepage[TOC caption]{picture}{caption}{label}
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{picturepage}[2012/04/16 v0.1 Picture Page]
\newlength{\maxpicturewidth}
\newlength{\maxpictureheight}
\newlength{\maxfigureheight}
\newlength{\pictureheight}
\newlength{\picturewidth}
\newlength{\totalfigureheight}
\newlength{\captionheight}
\newlength{\originalpicturewidth}
\newlength{\originalpictureheight}
\newlength{\picturehoffset}
\newlength{\picturevoffset}
\newlength{\vmargin}
\newlength{\subpictureskip}
\setlength{\subpictureskip}{0pt}
\newsavebox{\captionbox}
\newcounter{recalccounter}
\RequirePackage{pdflscape}
%\RequirePackage{lscape}
\RequirePackage{graphicx}
\RequirePackage{ifthen}
\RequirePackage{calc}
\RequirePackage[absolute]{textpos}
\RequirePackage{geometry}[2010/09/12]
\RequirePackage{afterpage}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%% \picturepage[ToC caption]{file}{caption}{label} %%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\newcommand{\picturepage}[4][\empty]{%
%\afterpage{
\clearpage
\thispagestyle{empty}%
\settowidth{\originalpicturewidth}{\includegraphics{#2}}%
\settoheight{\originalpictureheight}{\includegraphics{#2}}%
% check if we have a landscape picture
\ifthenelse{\lengthtest{\originalpicturewidth > \originalpictureheight}}{%
%%% landscape %%%
\newgeometry{textheight=\paperheight,textwidth=\paperwidth}%
\begin{landscape}%
\begin{figure}%
% set max picture dimensions
\setlength{\maxpictureheight}{.9\paperwidth}% TODO: make width/height depend
\setlength{\maxpicturewidth}{.9\paperheight}% on orientation one day
\setlength{\maxfigureheight}{\maxpictureheight}%
% calculate picture and caption dimensions including caption until it fits
\setlength{\captionheight}{\baselineskip}%
\setcounter{recalccounter}{0}%
\setlength{\totalfigureheight}{1.01\maxfigureheight+1pt}% force 1st iteration
\whiledo{\lengthtest{\totalfigureheight>1.01\maxfigureheight} % give some leeway
\AND \value{recalccounter}<10}{% if we don't converge in 10 iterations, give up
\ifthenelse{\value{recalccounter}>0}{%
\typeout{picture too high, recalculating (iteration \arabic{recalccounter})}%
}{}%
% calc initial picture dimensions
\setlength{\maxpictureheight}{\maxfigureheight-\captionheight-\subpictureskip}%
\settoheight{\pictureheight}{\includegraphics[width=\maxpicturewidth,%
height=\maxpictureheight, keepaspectratio=true, draft]{#2}}%
\settowidth{\picturewidth}{\includegraphics[width=\maxpicturewidth,%
height=\maxpictureheight, keepaspectratio=true, draft]{#2}}%
% measure caption height with initial picture dimensions
\savebox{\captionbox}{\parbox{\picturewidth}{%
\ifthenelse%
{\equal{#1}{\empty}}%
{\caption{#3}}%
{\caption[#1]{#3}}%
}%
\addtocounter{figure}{-1}%
}%
\settoheight{\captionheight}{\usebox{\captionbox}}%
% measured magic factor, total weirdness, should *so* be 1.0!
\setlength{\captionheight}{1.9\captionheight}%
% calc intial figure height including caption
\setlength{\totalfigureheight}{\pictureheight+\captionheight+\subpictureskip}%
\stepcounter{recalccounter}%
}%
\centering%
\includegraphics[width=\maxpicturewidth,%
height=\maxpictureheight, keepaspectratio=true]{#2}\vspace{\subpictureskip}
\usebox{\captionbox}\label{#4}%
\end{figure}%
\end{landscape}%
\restoregeometry
}{%%% portrait %%%
\setlength{\vmargin}{\topmargin}
\addtolength{\vmargin}{1in}
\addtolength{\vmargin}{\headheight}
\addtolength{\vmargin}{\headsep}
\addtolength{\vmargin}{\topskip}
\begin{figure}%
% set max picture dimensions
\setlength{\maxpictureheight}{.9\paperheight}%
\setlength{\maxpicturewidth}{.9\paperwidth}%
\setlength{\maxfigureheight}{\maxpictureheight}%
% calculate picture and caption dimensions including caption until it fits
\setlength{\captionheight}{\baselineskip}%
\setcounter{recalccounter}{0}%
\setlength{\totalfigureheight}{1.01\maxfigureheight+1pt}% force 1st iteration
\whiledo{\lengthtest{\totalfigureheight>1.01\maxfigureheight} % give some leeway
\AND \value{recalccounter}<10}{% if we don't converge in 10 iterations, give up
\ifthenelse{\value{recalccounter}>0}{%
\typeout{picture too high, recalculating (iteration \arabic{recalccounter})}%
}{}%
% calc initial picture dimensions
\setlength{\maxpictureheight}{\maxfigureheight-\captionheight}%
\settoheight{\pictureheight}{\includegraphics[width=\maxpicturewidth,%
height=\maxpictureheight, keepaspectratio=true, draft]{#2}}%
\settowidth{\picturewidth}{\includegraphics[width=\maxpicturewidth,%
height=\maxpictureheight, keepaspectratio=true, draft]{#2}}%
% measure caption height with initial picture dimensions
\savebox{\captionbox}{\parbox{\picturewidth}{%
\begingroup%
\ifthenelse%
{\equal{#1}{\empty}}%
{\caption{#3}}%
{\caption[#1]{#3}}%
\endgroup%
}%
\addtocounter{figure}{-1}%
}%
\settoheight{\captionheight}{\usebox{\captionbox}}%
% measured magic factor, total weirdness, should *so* be 1.0!
\setlength{\captionheight}{1.9\captionheight}%
% calc intial figure height including caption
\setlength{\totalfigureheight}{\pictureheight+\captionheight}%
\stepcounter{recalccounter}%
}%
\setlength{\picturevoffset}{\paperheight-\totalfigureheight}%
\setlength{\picturevoffset}{.5\picturevoffset}%
\setlength{\picturehoffset}{\paperwidth-\picturewidth}%
\setlength{\picturehoffset}{.5\picturehoffset}%
\begin{textblock*}{\picturewidth}(\picturehoffset,\picturevoffset)
\centering
\includegraphics[width=\maxpicturewidth,%
height=\maxpictureheight, keepaspectratio=true]{#2}\vspace{\subpictureskip}
\usebox{\captionbox}\label{#4}%
\end{textblock*}
\null\clearpage
\end{figure}%
}%
\addtocounter{figure}{1}%
\clearpage%
}%}
