One of the many potential themes that you could comment on in response to this question is that of class. Essentially, this is a story of a young woman who is made to realise the importance of class in spite of her desire to live her life as if it was not important. You might wish to consider how the character of Laura and her impressions are presented. Consider the following quote:

And now there came the chock-chock of wooden hammers. Some one whistled, someone sang out, "Are you all right there, matey?" "Matey!" The friendliness of it, the--the--

This quote shows how Laura looks favourably on the working class men as they ready the garden for the party. Imagery is of course best when it combines as many of the senses as possible, and here we have the sense of hearing combined with Laura's sight of these working class men. Imagery is used deliberately to present them as being pleasant and positive, which helps highlight the theme of class in this story. 

Of course, in spite of this, you might want to go on to explore the way in which the story presents Laura as being unable to escape from class conventions, no matter how much she may like to. Hope this helps!

What thesis shows how imagery reveals themes in "The Garden Party"?

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.