I know there are other ways to do it but it could be very convenient if it worked like that.
Not sure I understand what you mean, sorry?
The "Convert" button on the "Images Toolbar" doesn't seem to support converting the thumbnail of the selected video into a separate image in the desired format.
For example, selecting a file called "example.mp4", then using the button to convert the thumbnail of the video into a png named "example.png" as a separate file.
I'm bad at describing things 