Here's an example widget.xml file

<?xml version="1.0" encoding="utf-8"?>
  <name>My First Widget</name>
  <image usage="dock" src="images/SmallSun.png"/>
  <image usage="security" src="images/SecuritySun.png"/>
  <author name="Gregor Samsa" organization="Roach Motel" href=""/>
  <copyright>(c) 2006-2007 Gregor Samsa</copyright>
  <description>Gimmie a break, this is my first widget!</description>
  <platform minVersion="4.0" os="macintosh"/>

identifier is a unique identifier. Use a tool like UUID Vault to generate an ID.

the dock image is a 75x70 image. If you're not already doing stuff with the dock, you should at least put an image there so your widget doesn't get slapped with a generic icon. Dock image doesn't have to be 75x70.

the security image is a 150x150 image. It is the image that users see when the security window pops up.

the platform restricts which engine and which OS you'll let your widget run on.

More information on widget.xml, version, and identifier can be found here: Widget Update for All!