OpenAI has published no measurement per image. The open generators tested in the laboratory use 2.9 Wh per image on average, or about 1.3 gCO2e with global average electricity, the equivalent of 9 metres by car. Published measurements range from 0.04 to 5 gCO2e, and nothing shows whether ChatGPT's generator falls within that range.
Read the source article: AI image generation carbon footprint: 1.3 gCO2e on average