All you need is a linux distro with ffmpeg installed, a jpeg or png image, and an mp3. Note: It is highly likely other image formats, audio formats, and output video formats will work, but I have only used jpeg/png+mp3+avi and so cannot attest to results otherwise.

Usage: bash mp32avi.sh <image_file> <mp3_file> <output_file.avi>

Code (mp32avi.sh):

#!/bin/bash
FFMPEG=`which ffmpeg`
if [ "$FFMPEG" = "" ] ; then
	echo "Please install ffmpeg.";
	exit 0;
fi
if [ $# != 3 ] ; then
	echo "Usage: $0 <image_file> <mp3_file> <output_file.avi>";
	exit 0;
fi
if [ ! -f $1 ] ; then
	echo "Source image '$1' not found.";
	exit 0;
fi
if [ ! -f $2 ] ; then
	echo "Source mp3 '$2' not found.";
	exit 0;
fi
if [ -f $3 ] ; then
	echo "Output file '$3' exists.  Overwrite? (y/n)";
	read CONFIRM
	if [ "$CONFIRM" == "y" ] ; then
		echo "Overwriting '$3'"
	else
		if [ "$CONFIRM" == "Y" ] ; then
			echo "Overwriting '$3'"
		else
			echo "Operation canceled.";
			exit 0;
		fi
	fi
fi
TIME=`$FFMPEG -i $2 |& grep 'Duration' | awk '{ print $2; }' | sed -e 's/,//g'`
$FFMPEG -loop_input -i $1 -i $2 -acodec copy -y -t $TIME $3