Review
If you allow thumbnail submission and hotlinking and thumbnail and hotlink were submitted, at the left side of review form you will see submitted thumbnail, "Browse" button and checkboxes "M" and "A", below you will see hotlinked thumbnail and "Hot" checkbox:
If you don't like thumbnail, you can create new one and upload it.
Checked box "M" means you want use thumbnail on the main page.
Checked box "A" means you want use thumbnail on archives pages.
Checked box "Hot" means you want use hotlinked thumbnail.
TGP Brownie thumbpreview help