% Declarations
% -----------------------------------------------------
  \newdimen\hoffset
  \newdimen\voffset
  \newcount\howwide
  \newcount\howhigh
  \newdimen\pagewidth
  \newdimen\pageheight
  \newdimen\overlap
  \newdimen\totalwidth
  \newcount\hoffsetcounter
  \newcount\voffsetcounter
  \newcount\totaloffsetcounter
  \newcount\totaloffsets

% Set the operational parameters. How many pages wide
% should the output be, and what is the overlap between
% pages? Use any unit: 20mm, 1in, -5pt, whatever.
% -----------------------------------------------------
  \howwide=5
  \overlap=15mm

% You can play with the page height/width all you want.
% Here they're set to landscape A3.
% -----------------------------------------------------
  \pagewidth=420mm
  \pageheight=297mm
  \pdfpagewidth\pagewidth
  \pdfpageheight\pageheight

% Vanity ;-)
% -----------------------------------------------------
  \pdfinfo {
      /Title    (Blowup)
      /Creator  (blowup)
  }

  \hoffset=\pagewidth
  \voffset=\pageheight
  \advance\hoffset by -\overlap
  \advance\voffset by -\overlap
  \totalwidth=\hoffset
  \multiply\totalwidth by \howwide
  \advance\totalwidth by \overlap
  
  \pdfximage width \totalwidth {YOUR-FILE-HERE.png}
  \newbox\picbox
  \setbox\picbox=\hbox{\pdfrefximage\pdflastximage}
  
  \howhigh=\ht\picbox
  \divide\howhigh by \voffset
  \advance\howhigh by 1
  
  \hoffsetcounter=0
  \voffsetcounter=0
  \totaloffsetcounter=0
  \totaloffsets=\howwide
  \multiply\totaloffsets by \howhigh
  
  \loop
      \pdfhorigin -\hoffset
      \pdfvorigin -\voffset
      \multiply\pdfhorigin by \hoffsetcounter
      \multiply\pdfvorigin by \voffsetcounter
      \shipout\copy\picbox
      \advance\totaloffsetcounter by 1
      \advance\hoffsetcounter by 1
      \ifnum\hoffsetcounter=\howwide
          \advance\voffsetcounter by 1
          \hoffsetcounter=0
      \fi
      \ifnum\totaloffsetcounter<\totaloffsets\repeat
  
  \end