scrlttr2 - Using different background images for 1st page

Question

I want to use different background images within a letter I created with scrlttr2, on the first page image A and on all other pages image B.

Here is a working minimal example to use the same background on all pages.

Unfortunately I have no idea how to change this so that it works the way I described it.

\documentclass[%%
    fontsize=12pt,
    paper=a4,
    DIV=15,
    version=last
]{scrlttr2}

\usepackage{fontspec}
\usepackage{microtype}
\usepackage{graphicx}
\usepackage{eso-pic}
\usepackage{blindtext}

\newcommand\BackgroundPic{
    \put(0,0){
        \parbox[b][\paperheight]{\paperwidth}{%
            \centering
            \includegraphics[width=\paperwidth,height=\paperheight]{picture.pdf}
}}}

\begin{document}

\AddToShipoutPicture{\BackgroundPic}

\begin{letter}{Address}
\setkomavar{subject}{Letter}
\opening{Sir}
\Blindtext

\end{letter}
\end{document}

Regards, Robert

Answer 1

You can load package scrlayer-scrpage, declare a new layer and add this layer to the special layer page style @everystyle@ (see the documentation).

\documentclass[%
    %fontsize=12pt,% default
    %paper=a4,% default
    DIV=15,
    %version=last% default
]{scrlttr2}
\usepackage{blindtext}% only for dummy text
\usepackage{fontspec}
\usepackage{microtype}
\usepackage{graphicx}

\usepackage{scrlayer-scrpage}
\DeclareNewLayer[
  background,
  mode=picture,
  contents=\putLL{%
    \ifnum \value{page}=1
      \includegraphics[width=\layerwidth,height=\layerheight]{example-image-a}%
    \else
      \includegraphics[width=\layerwidth,height=\layerheight]{example-image-b}%
    \fi
  }
]{letterbackground}

\AddLayersToPageStyle{@everystyle@}{letterbackground}

\begin{document}
\begin{letter}{Address}
\setkomavar{subject}{Letter}
\opening{Sir}
\Blindtext[10]
\end{letter}
\end{document}

Result:

It is also possible to have more than one letter in a document:

\documentclass[
    %fontsize=12pt,% default
    %paper=a4,% default
    DIV=15,
    %version=last% default
]{scrlttr2}
\usepackage{blindtext}% only for dummy text
\usepackage{fontspec}
\usepackage{microtype}
\usepackage{graphicx}

\usepackage{scrlayer-scrpage}% sets page style scrheadings automatically
\DeclareNewLayer[
  background,
  mode=picture,
  contents=\putLL{%
    \ifnum \value{page}=1
      \includegraphics[width=\layerwidth,height=\layerheight]{example-image-a}%
    \else
      \includegraphics[width=\layerwidth,height=\layerheight]{example-image-b}%
    \fi
  }
]{letterbackground}

\AddLayersToPageStyle{@everystyle@}{letterbackground}%

\begin{document}
\begin{letter}{Address}
\setkomavar{subject}{Letter}
\opening{Sir}
\Blindtext[10]
\end{letter}
\begin{letter}{Address}
\setkomavar{subject}{Letter}
\opening{Sir}
\Blindtext
\end{letter}
\end{document}

---

It is also possible to declare a layer for the first letter page and another for next pages:

\documentclass[%
    %fontsize=12pt,% default
    %paper=a4,% default
    DIV=15,
    %version=last% default
]{scrlttr2}
\usepackage{blindtext}% only for dummy text
\usepackage{fontspec}
\usepackage{microtype}
\usepackage{graphicx}

\usepackage{scrlayer-scrpage}
\DeclareNewLayer[
  background,
  mode=picture,
  contents=\putLL{\includegraphics[width=\layerwidth,height=\layerheight]{example-image-a}}
]{letterbackground.firstpage}
\DeclareNewLayer[
  background,
  mode=picture,
  contents=\putLL{\includegraphics[width=\layerwidth,height=\layerheight]{example-image-b}}
]{letterbackground.nextpage}

\AddLayersToPageStyle{empty}{letterbackground.firstpage}% first letter page uses page style empty
\AddLayersAtBeginOfPageStyle{scrheadings}{letterbackground.nextpage}

\begin{document}
\begin{letter}{Address}
\setkomavar{subject}{Letter}
\opening{Sir}
\Blindtext[10]
\end{letter}
\begin{letter}{Address}
\setkomavar{subject}{Letter}
\opening{Sir}
\Blindtext
\end{letter}
\end{document}

The result is the same as above.

---

Additional suggestion using eso-pic:

\documentclass[
  %fontsize=12pt,% default
  %paper=a4,% default
  DIV=15,
  %version=last% default
]{scrlttr2}

\usepackage{fontspec}
\usepackage{microtype}
\usepackage{graphicx}
\usepackage{eso-pic}
\usepackage{blindtext}

\newcommand\BackgroundPic{
  \put(0,0){% <- needed to avoid a spurious space
    \parbox[b][\paperheight]{\paperwidth}{%
        \centering
        \ifnum \value{page}=1
          \includegraphics[width=\paperwidth,height=\paperheight]{example-image-a}%
        \else
          \includegraphics[width=\paperwidth,height=\paperheight]{example-image-b}%
        \fi
}}}
\AddToShipoutPicture{\BackgroundPic}

\begin{document}
\begin{letter}{Address}
\setkomavar{subject}{Letter}
\opening{Sir}
\Blindtext[10]
\end{letter}
\begin{letter}{Address}
\setkomavar{subject}{Letter}
\opening{Sir}
\Blindtext
\end{letter}
\end{document}