You could probably create a basic text column, but don’t fill it with anything. So an empty column. Then create a template column that’s pointed to the url column and replaces any parts that don’t contain the video id with the empty column. The end result should be the id that can be used to build the url for the image.
As an alternative, you could create a template column pointing to the url column and instead of replacing non-id parts of the url with an empty value, you directly replace the non-id parts with the parts of a url required for the image url. This will require separate template columns with partial urls to use for the replacements.