What is the requirement for the images in the slider?

The plugin adds three cropped thumbnail sizes for use upon activation.  These are produced when images are uploaded to the media library.  Their sizes are: 540x388px, 540x192px, and 270x192px.  If your slider is not aligned as in the example, you may need to re-upload the featured images for the posts displayed. Or, use a thumbnail resizing plugin to do this all at once.  Of course, the latter option should be chosen only if the image library is small.