This is something I get asked A LOT. It seems to be a thing that really paralyses a lot of first-time comics writers, particularly ones coming from other media. What is the picture? How do I find the right panel to describe?
A useful starting place might be something the actor and comics writer Nick Vince said, back in the early 90s. It comes from cinema, as did Nick, and it goes like this: imagine the panel as your “print moment.” The frame that captures the essence of the moment. Imagine, say, thirty seconds’ worth of film, and that your job is to overlay those thirty seconds of dialogue over a single frame pulled from that ribbon of film that best encapsulates what’s going on.
That’s a very mechanical way of looking at it, but it might get you started. You’re looking for the image that captures the moment.
You’re also, wherever possible, looking for an interesting image. But don’t confuse “interesting” with “splashy.” You’re still trying to serve the demands of storytelling, telling the story as clearly and simply as possible. In most forms of narrative, each panel must have a relationship with the panels on either side of it. You’re plotting out a sequence of motion in a series of stills. Imagine it like that, and you may be able to get a better sense of how a story in comics might flow. It’s not a perfect analogy, but it might be worth considering if this is something you’re having trouble with. You’ll develop your own view, approach and methods as you go. Everybody does.