A strong thesis statement is a critical part to writing a cohesive essay! There are three essential components to a thesis statement:
1. The subject (informs the reader who or what the essay will focus on)
2. The precise opinion (the argument that you will advance in the essay; this should be something that is "contestable", i.e. one could develop a counter-argument for it)
3. A "blueprint" of your reasoning (this should outline the lines of reason that you will use to support your precise opinion). This "blueprint" will be used to guide the structure of your essay, and provides the reader with a short overview of your argument.
The thesis statement should be direct, avoid vague language, and clearly state the argument you will be making in the essay. Your proposed statement is well on the way to meeting these requirements; however, there are a few aspects that could strengthen it. First, I suggest a stronger opening to the sentence by stating who is employing the imagery. For example, "In the short story 'Boys and Girls' by Alice Munro, imagery is used to..." Further, I would change the words used to describe your subject. I assume that "the girl" refers to the story's narrator. Therefore refer to her as the narrator in your thesis statement, in order to convey the importance of her role in the story to the reader of your essay. To build upon the previous example, "In the short story 'Boys and Girls' by Alice Munro, imagery is used to demonstrate how the narrator is forced to conform to defined gender roles".
Finally, I suggest strengthening the "blueprint" of your thesis statement. While you assert that imagery is employed as a literary device to convey the influence of gender roles, it would be helpful to give an overview of what kind of imagery is used, and how. This does not have to be extensively detailed, just a quick identification of the type imagery is used by the author, and its role in the story. The rest of your essay will then build upon this "blueprint". For example, "In her short story 'Boys and Girls', author Alice Munro used [X], [Y], [Z] imagery to demonstrate how the narrator is forced to conform to defined gender roles". This highlights the subject of your essay, states your argument, and guides the reader into your line of reasoning.