The November 4 OpenAI changelog added markup-guided editing for supported GPT Image 2.5 and 2.6 models. Use the Image API's POST /v1/images/edits. The original image stays clean. Create a second file, a transparent PNG, with the same dimensions as the oriented first image. Draw your marks there and send the PNG as markup. This edit accepts either markup or a mask.
Make one change easy to inspect
For a fictional event poster, circle door number 24 on the overlay. In the prompt ask for 26 in the circled position, with the same size and alignment. Ask to preserve the border and neighboring labels. A 1024 × 1024 original and an equally sized overlay fit the documented pixel limit. API access and charges depend on your organization and model.
Compare the result with the original
Zoom in on the new number, the border and one unmarked corner. Check that the number reads 26 and that the mark itself and unintended changes are absent. If the poster needs exact typography, set the number manually and verify the final export.