You may create a list of thumbnail options that users can choose from.
<ul>
  <li>Use alphanumeric characters as thumbnail names.</li>
  <li>Specify dimensions as <strong>WidthxHeight</strong>.</li>
  <li>Prefix and suffix are strings that are added to original file name to create the thumbnail name.</li>
  <li>An example thumbnail: Name = <strong>Small</strong>, Dimensions = <strong>80x80</strong>, Prefix = <strong>small_</strong></li>
</ul>
<p>Note that users will always be able to create these thumbnails on file upload no matter what the thumbnail permission is.</p>
<p>If you need more fields, just fill all and save, and you will get two more on the next page.</p>