It sounds as if you already have a number of excellent ideas to start writing your thesis statement with. Key for me is the way in which the description we are given of the wallpaper at the start of this brilliant story corresponds very closely with the mental state of the narrator herself. In a sense, she could be said to project her own condition on to the wallpaper. Consider the following description that we are given:
It is dull enough to confuse the eye in following, pronounced enough to constantly irritate and provoke study, and when you follow the lame uncertain curves for a little distance they suddenly commit suicide--plunge off at outrageous angles, destroy themselves in unheard-of contradictions.
Note the way that the use of words such as "commit suicide" and "outrageous angles" and "unheard-of contradictions" clearly says something very profound about the mental state of the narrator and the way that she is on the verge of committing some form of violence against herself. A good thesis statement for your prompt therefore might be:
The mental stability of the narrator is clearly indicated in the way that the yellow wallpaper is described and how this description develops through the story.
This would allow you to explore how the yellow wallpaper develops in terms of its imagery and symbolism, in particular with the vision of the women who is trapped behind its "bars," and also you can relate this development to the mental state of the narrator.