Mirrors Meta's POST /{accountId}/adimages.

Body carries Meta's own ad-image-upload fields — a bytes base64-encoded field — and is forwarded verbatim. Returns the image hash used by object_story_spec.link_data.image_hash on creative create.

• Write — changes a live ad account immediately; needs a write-scoped token.

Uploading a file. As well as the JSON body above, this route accepts multipart/form-data with a file part (other parts ride along as platform fields), or file_base64 inside the JSON body — raw base64 or a data: URI, plus optional file_name/file_type. file_base64 is the only way an MCP caller can send bytes. JPEG, PNG, GIF or WebP, up to 10MB. The bytes are base64-encoded in place and sent to Meta as bytes — nothing is staged in object storage, because adimages has no URL parameter. A rejected type or size is 400 FILE_UPLOAD_INVALID.