How to add Photos in background to a Video using FFMPEG
I'm going to call the video/image on top, overlay, and the bottom one, the photo.

ffmpeg -i photo -i overlay
       -filter_complex "[1]split=2[color][alpha];
                        [color]crop=iw/2:ih:0:0[color];[alpha]crop=iw/2:ih:iw/2:0[alpha];
                        [color][alpha]alphamerge[ovrly];
                        [0][ovrly]overlay=0:0" output.jpg


I assume that if the resolution of the video is WxH then the resolution of the photo is W/2 X H. If it's not, add a crop filter to the photo and then use that result in the overlay.

If the resulting video has uneven dimensions, add a scale filter after the overlay to rectify that: [0][ovrly]overlay=0:0,scale=2*trunc(iw/2):2*trunc(ih/2)



If you have a sequence of images of the same resolution, then you can use the concat demuxer.

First, create a text file,

file 'image1'
duration 2.0
file 'image2'
duration 4.0
file 'image3'
duration 2.0


Then,

ffmpeg -f concat -i text -i overlay
       -filter_complex "[0]fps=25[bg];
                        [1]split=2[color][alpha];
                        [color]crop=iw/2:ih:0:0[color];[alpha]crop=iw/2:ih:iw/2:0[alpha];
                        [color][alpha]alphamerge[ovrly];
                        [bg][ovrly]overlay=0:0,scale=2*trunc(iw/2):2*trunc(ih/2)" output.jpg