Making A Movie Poster Question

An independent filmmaker has completed a feature and wants me to make a movie poster, using footage from the film.

I told him I would need TIF files for quality sake, so he sent me a hundred or so, and I played around with them.

But I came to realize that while they were TIF files, they were really screen grabs.

I learned that there's a program called DaVinci Resolve 11 which sells for $29,995, but they also have a free download which will, I understand, get me the high resolution files.

I downloaded it but it will only run on Windows 64 bit, which I don't have.

My question is this: do any of you know of a program that will copy digital files from a movie and give me high resolution files?

Chuck Staley Concept Art