An easy way to create videos for YouTube from mp3 files
Thanks to Brads recommendation, I used ffmpeg for the job. Here is a small bash script that converts all the mp3 files to mpeg4 avi files (a jpg image must be included in the folder):

#!/bin/bash
# Requires: ffmpeg
# Usage: ./convert.sh

for file in *.mp3;
do
    ffmpeg -i *.jpg -i "${file}" "${file%.*}.avi"
done


Some more info here and here, but that didn't work for me.. All these options they used made it too complicated and it just didn't work - nothing would get converted (I would get files with audio but without the image in the video or videos with image but without audio, -loop option was unnecessary, -shortest option was unnecessary, works fine without it)..