Hi! I just checked out your game and left a review.
I don't have much experience with text-based games, but I actually think your game is a great candidate for GIFs. You could show the main mechanic of clicking on words to reveal associations, which gradually uncover more context about the story.
For screenshots, I'd add one for each major part of the UI. For example, one showing the main text, one for the letter, and maybe one for the How to Play screen.
You could also create screenshots that hint at the themes of grief and a troubled family. For example a screenshot showing the words "Ma and Pa wanted to protect me more than anything", another that shows something about the gambling debt problem, another about mourning a loss (if you're ok with some spoilers), etc.
Great game, by the way!