It sounds like you do not have a drawing program and you have never used any drawing program, so there is probably not enough space in the forum to write out all of the thing you need to learn to accomplish what you are asking. Your best bet is to ask someone else to make your thumbnail image for you.
If you want to make the image yourself:
1) pick a drawing program such as GIMP (free at gimp.org) and install it on your computer
2) learn how to use the simplest features of the program (using tutorials online)
3) take a screenshot of your videoscribe project and open it in your drawing program
4) follow any tutorial that explains how to add text over your screenshot, then save it.
-Mike (videoscribe user)
VideoScribe is software for making whiteboard animations. You may be able to produce a transparent thumbnail but it may depend on what you would be using the thumbnail for as to the suitability of the software for that kind of a job.
Could you give more detailed explanation of what you are trying to achieve and what you would like to use it for?
That is not something you can do in videoscribe. Search the internet for a tutorial that explains how to do it in your preferred program.
For example: adding text and shapes using photoshop cc
Could you not create the file as a transparent .png file and place in videoscribe?
I think you would need to upload your scribe video onto YouTube and then make the video 'interactive' by adding an overlay. See About interactive video ads on the YouTube support site for more info.