That study is from 2023 when the cost to produce 1000 images was 2.91 Wh per image. More recent numbers for Stable Diffusion's 2025 models that puts it at 1.3 Wh per image and that number continues to decrease.

For context that means a days worth of image generation emits about the same amount of carbon dioxide as a single transatlantic flight (New York to London). There are approximately 1500 transatlantic flights per day.