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}
