Your answer to this question of course depends on which theme you wish to focus on in your response. For me, the central theme is based on the epiphany that Laura experiences at the end of the story whilst looking at the dead body of Mr. Scott. Note how his corpse is described and what imagery is used to emphasise the appearance of his dead body:
There lay a young man, fast asleep--sleeping so soundly, so deeply, that he was far, far away from them both. Oh, so remote, so peaceful. He was dreaming. Never wake him up again.
The imagery employed here is deliberately designed to emphasise the death as something that is beautiful and bestows upon the poor Mr. Scott a nobility and wisdom that he perhaps never had in life thanks to his low class. The imagery then thus serves to emphasise the theme of the contrast between the dead Mr. Scott and Laura's frivolous concerns with the Garden Party of her family and her new hat. A suitable thesis statement therefore might be:
In "The Garden Party," imagery is used to elucidate the themes through the presentation of the body of Mr. Scott.
This thesis statement would allow you to focus in on the way that Mansfield uses imagery to present the contrast between Laura and Mr. Scott, which then leads you to a discussion of the themes she conveys. Hope this helps! Good luck.