Can not add images after add background to audio - ffmpeg

Question

I use below command to concat background to an audio file:
"ffmpeg" -i /path/to/image.png -i /path/to/audio.mp3 -vsync vfr -pix_fmt yuv420p /path/to/video-1.mp4 2>&1
Next, I use that command to add other images to created video:
"ffmpeg" -f concat -safe 0 -i /path/to/text.txt -i /path/to/video-1.mp4 /path/to/video-2.mp4 2>&1
Content of my text.txt file:
/path/to/img1.jpg
duration 6
/path/to/img2.jpg
duration 6
/path/to/img3.jpg
duration 6
/path/to/img4.jpg
duration 6
Obtained video display only background image. Other images have not been shown.
What are my wrong in these commands? And how to set the display position of other images in video? To display images by position, I use below command, but after it ran, my machine was crashed:
"ffmpeg" -f concat -safe 0 -i /path/to/text.txt -i /path/to/video-1.mp4 -filter_complex "overlay=0:0, scale=640:640" /path/to/video-2.mp4 2>&1
Update:
I do follow order by below steps and got video with text and images, but the position is incorrect. I can't find any solution for custom:
"ffmpeg" -f concat -safe 0 -i /path/to/text.txt -i /path/to/audio.mp3 -vsync vfr -pix_fmt yuv420p /path/to/tmp-video-1.mp4 2>&1
"ffmpeg" -i /path/to/background.png -i /path/to/tmp-video-1.mp4 -filter_complex "overlay=0:0" /path/to/tmp-video-2.mp4 2>&1
"ffmpeg" -i /path/to/tmp-video-2.mp4 -vf "[in]<define texts and duration>[out]" -codec:a copy /path/to/endvideo.mp4 2>&1
Please take a look my video: http://184.171.170.45/cron/tmp/funny-1489648654.mp4
I tried to set overlay property in second command: -640:0, let images in horizontal center of background, but its not working