To set custom thumb for your video (or audio) file do the following :
- Upload Video at https://publit.io/dashboard/files
- Upload Image you want to use as a custom thumb at https://publit.io/dashboard/files
- Edit Video by clicking Edit under the "..." (more options) and going to file edit page
- 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