You should probably contact CGTrader support for a definitive answer. However, just a quick glance at your photos seems to indicate your previews are not specified in the correct aspect ratio for display here. Usually, you will want your preview renders to be in a 16:9 aspect ratio, 1080p, or better resolution. Your previews appear to be a 4:3 aspect ratio, but I could be wrong there. I'm not sure why they are rotating, but perhaps the dimensions of the images have something to do with it? Let us know what the official word is on this when you get an answer. I'll be curious to know.