Impossible to fine-tune ChatGPT-4o-mini with Base64 images

I tried sending a JSONL file formatted like this:

{"messages":[{"role":"system","content":"System image instructions"},{"role":"user","content":[{"type":"text","text":"Can you explain this image?"},{"type":"image_url","image_url":{"url":"data:image/png;base64,datas_B64_image"}}]},{"role":"assistant","content":"Yes, this image represents a kitten, ...."}]}

But I keep encountering a “Failed” error…

Thank you for your help!