Display your recent comments the way YOU like and use fantastic looking comments by default!
The gravatar default url field is optional and provides the possibility to link to a static image on your server. This is might be important if you have many comments and are concerned about the page loading times because the gravatar feature loads the default image from the gravatar servers which can slow down the progress.
You have to know a bit about html and css to style the output of the plugin. There are two options: You can use inline styles directly in the output format field (with: style="...") or you can use your standard wordpress css files and add rules at the end of them to modify the appearance of the output (this might be complicated for beginners). I am working hard to provide you with an easier solution as soon as possible.