If you are referring to the List Category Posts plugin, it uses WordPress’ native function to get the thumbnail get_the_post_thumbnail which produces:
class="attachment-thumbnail wp-post-image"
You should assign style to those classes. More info about that here.
I’ll be adding the thumbnail CSS class as a parameter since get_the_post_thumbnails has that as an optional parameter, so you’ll have better customization on a future release.
UPDATE: Ok, now I added the functionality to send the CSS class as a parameter for your thumbnail:
thumbnail_class – Set a CSS class to the thumbnail and style it.
This is working as of version 0.21 of List Category Posts.