You can just use the one insert page to upload the image as well. Thumbnails can be created automatically using the same image combined with the Image Resizer extension which is NOT part of CSS Form Builder 2 but is included in Data Bridge. It can be purchased separately at the moment but you might be better off upgrading to Data Bridge. Get in touch with sales, tell them the extensions you have and they will give you an upgrade price. email@example.com
You can also, if you wish upload two images (Full size and thumbnail) using the one insert page. You don't HAVE to use "Image Resizer"