Create Image Thumbnails

I am currently loading images into an object via the Knack API and having an issue where the thumbnails are not created through this process.

The field I am loading into is an image field and I have the thumbnail generation option selected with both width and height set.

Is there an option I need to pass in via the API to create/use an image as a thumbnail?

Is there a way to have Knack generate a thumbnail when loading via the API?

If I instead host images externally and set the Image field to be a URL will it then render/show a thumbnail?

Thanks for the help.