Sorry about this, it's part of the user interface I never fully finished so it's incredibly confusing.
In the meantime, the format can be set in "configure field settings" with a line for each choice containing comma separated values:
id,name,description,image_url
The image_url and description are optional. id is a unique string that you can use to preserve responses if you decide to re-order or re-name them. You can just use an incrementing number:
1,First choice,Description about first,
2,Second choice,Description about second,
3,Another choice,Etc.,