You have asked the same question in another topic. And I have answered it, so please don't mind if I copy my answer here:
If images are seem to be displayed incorrectly it usually means that preview images or thumbnails are smaller (or larger) by size than the Gallery itself. In order to make your Gallery look better please set appropriate size for preview and thumbnail images. Please choose optimal sizes to avoid the images to be displayed incorrectly in your Gallery. You can compare them to the images which template includes by default, they usually have optimal dimension for the template galleries.