Video preview looks grainy

Hi. I recently posted question on why illustration quality drops drastically when I post it to Rise and I managed to solved it by changing the format to .svg. However, now I’m encountering similar issue with videos. As shown in picture below when I post a video which includes preview picture it looks grainy and like it lacks color pixels (kind of like a gif would). When I play the video the quality is fine. I tried rendering the video in few different ways but it always looks the same. Is there any way to avoid this since I’ve seen it done in template courses and previews look very clean and nice?

Thanks