Here's the websites I use. No idea where you're getting your free material because all these are at least Full HD:
Pexels.com
Pixabay.com
videvo.net
videezy.com
mixkit.co
coverr.co
mazwai.com
I think any modern editor can do that as long as it can output to the format you need. Just add a black still on the timeline and set the duration of it to one frame and then render.