embed an intro to videos using ffmpeg

I have a script that downloads and then uploads videos. I would like the videos before upload to have an intro screen added, that would show up for 5 seconds. I guess it should be an image and added using ffmpeg or similar program but it has to be lite. So the video should look like this - 5sec intro picture and then the whole video. What I need is help with the software instalation (like ffmpeg or similar) and the command for the ffmpeg (or similar) so the intro would be added and it should have some checks so it would not use much memory . Also all of the videos have different sizes and formats (not everyone is different, but there are plenty of possibilities). 30$ for this.