Using an API for Talks produced a square result

I'm trying to get the API to produce a 720 x 1280 video (which is the size of the image that is referenced):
"script" = {
"type" = "audio",
"audio_url" = form.audioUrl
},
"source_url" = form.imageUrl,
"config" = {output_resolution: 1280}

But the result is a video 512 x 512. What am I doing wrong? Thanks. Jay