All I want to do is put a still image over a MP3 so I can upload a song to Youtube. (Sidenote: It feels really good to find a song I want to show someone that isn’t already on Youtube. It used to be a somewhat regular thing i’d do, I have about a dozen Youtube videos that are just songs I uploaded because I wanted to show them to someone, but I guess Youtube got more stuff and my taste got more pedestrian, so I haven’t felt the need to do it until now. Feels good!)
I used VEED, a web editor, and it produced a >300mb file. That seems a bit excessive. For the curious, this is the song: https://youtu.be/iLz7VXhCrnk
I use Kdenlive for video editing. It’s Free Software. And you can use it for more complicated tasks, too. There are some other possibilities like OpenShot, Shotcut, or using a CLI tool like ffmpeg.