To set custom thumb for your video do the following :
1. Upload Video at https://publit.io/dashboard/files
2. Upload Image you want to use as a custom thumb at https://publit.io/dashboard/files
3. Edit Video by clicking Edit under the "..." and going to edit page at https://publit.io/dashboard/files/VIDEOID/edit
4. Look there for Thumbnail field, and search/select Image you want to use as a custom thumb. Save changes by clicking Update button at the end of page.
That's it. Your video will now have custom thumb set
Publitio handles uploading, hosting, processing, delivery of images, videos & audio files for web and mobile apps.