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}