probably need to update the model id and url in admin/plugins/discourse-ai/ai-llms . i believe gemini-2.5-flash-pre was deprecated this past January. they should update to gemini-3.1-pro-preview or one of the other ones listed here:
but by the looks of what you are asking it - that is to generate an image, i’d say you want a Nanobanana model (gemini-3.1-flash-image-preview).