How can I add a background image to a large music file? My music file is about ~300 MB and ~4 hours long. I'd like to add a background image onto it to upload it on Youtube. The problem is, when I try a video rendering program (Kdenlive, Windows Movie Maker etc.) to do that, the output becomes massively big, like ~40 GB. I don't understand, why does merging procession of a 300 MB mp3 file and a 1 MB png file result such a huge file size like that? Does the rendering program render the background image again and again for every frame? Can't the whole video have just a single frame and an audio attached on it?

Is there another way to accomplish it?
Simple answer is yes - the Video frame is repeatedly encoded multiple times. There is compression - including frame-to-frame, but the single image frame is effectively duplicated numerous times. Try reducing the resolution for lower file size - if the quality of that image is not so important.