Board Thread:General Discussion/@comment-17634531-20140819085700/@comment-5558012-20140819142242

There's no way to directly state a certain image to represent the article, ie choosing a file and stating to the software this image will be used. However, you can choose the image you want by meeting some criteria. The requirement that commonly disqualifies images is its size. The image must be at least 130 by 115px for category galleries (200 by 100px for the Read more section) on the file page. The second most common is that it can't be used more than ten times on the wiki (the limit can be raised by contacting Staff). Additionally, the first image to qualify in the wikitext is used regardless if it's in a template.

Also, you can't readjust the thumbnail directly, but you can crop your image and replace the current file until the thumbnail looks better.