I agree with an earlier answer that the place to start is with figuring out what you think the story means, and what the themes are. For me, the thing that is interesting about Hemingway is how, for all the focus on the directness and concreteness of his prose, most of the meaning of it is left unsaid. In the case of this story, it is mostly dialog, but in fact very little real communication is happening. There are many reactions one could have to these characters: Jig is being bullied, of course, but she is also fighting back in a passive aggressive way (“I don’t care about me,” she says when she gives in). There is an emptiness in their relationship; they are talking at each other, rather than to each other, and there is a kind of intractable quality to each of them. Someone described them as “decadent,” but to me these are people who realize that they have made a tremendous mistake but don’t want to admit it or face up to each other. I don’t mean the baby, but their lives.
So a thesis statement about symbolism in the story would need to account for this. I guess another way of thinking about a thesis would be to ask some questions: How does the setting of the story, or any of the descriptive details Hemingway provides, relate to this theme? Do these things contrast with this feeling of disconnectedness or reinforce it? Can these details be seen as ironic in any way? I think if you consider these questions, you can come up with a good thesis about how Hemingway uses the landscape as a way to comment on, and distance himself from, these characters.