To set a 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 the Edit icon on the left corner, and going to the file edit page
- In the Thumbnail field, search/select Image you want to use as a custom thumb. You can also add new thumbnail by clicking on + button
- Save changes by clicking the Update button at the end of the page.
That's it. Your video will now have a custom thumb set