Review

If you allow thumbnail submission, and thumbnail was submitted, at the left side of review form you will see thumbnail, "Browse" button and checkboxes "M" and "A":

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.

TGP Brownie thumbpreview help