I wrote my own tool for that a while back as an LLM plugin, so I can do this:

    llm cmd extract first frame of movie.mp4 as a jpeg using ffmpeg

I use that all the time, it works really well (defaulting to GPT-4o-mini because it's so cheap, but it works with Claude too): https://simonwillison.net/2024/Mar/26/llm-cmd/