Grab a Screenshot and Save It
If necessary, crop it down to 570px width.
Upload File
Choose the file form your hard drive. After the file is uploaded, copy the File URL. Then save your changes.
Enter the Image URL
Paste the url you copied for the image into the field. Then select Update.
Find Thumb Image
FTP into the server and find the thumbnail image.
Create a new thumb custom field
Create a new thumb custom field and enter in the thumbnail url.
Do the same for the thumbnail image