Podcast Producer will automatically generate a poster frame for each video that is generated. The poster frame is a frame of the video that is automatically generated and then applied to the video. The poster frame that each video has can therefore be a bit random. However, you can use any image that you like as a Poster frame (also called Poster Image in Podcast Producer).
To manually configure the poster frame, open the workflow in Podcast Composer (or do so during the initial creation process). Then click on the Window menu and click on Workflow Inspector (or use the Command-I shortcut key when you’re in Podcast Composer. At the resultant screen you will see a field for Episode Poster Image. Simply drag any image that you would like to use as a poster frame for all of the podcasts created from this workflow.
The poster frame will then appear in the wiki or in iTunes for each podcast generated using the workflow that you manually assigned a poster frame for.