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.
I'd just go for cropping. It's possible to do it more sophisticated with e.g. content aware fill in After Effects but the time it takes wouldn't weigh up to the results.