The quality of the cover image and the video preview of your items.

Hi, everyone! I am new to VideoHive. And I have some problems with the quality of the cover image and the preview video of my items. For the video preview I use a resolution of 1920x1080 the same goes for the cover image. On my computer everything looks perfect, but on VideoHive both the image and the video look bad quality. So, maybe there are some specific services or tools which I don’t know for optimizing video for the web? Maybe some specific resolutions for the cover image which will work the best way?
Any help, advice will be appreciated. Thanks in advance