ffmpeg thumbnailor with timecode in filename

Question

i have many videofiles and i want to make one thumbnails every 20 seconds. This is what i have now:

for %%a in ("*.*") do ffmpeg -i "%%a" -f image2 -vf fps=1/20 "index\%%~na_IMG-%%05d.jpg"

now i want to give the thumbnails a name with the timecode like this:

VHS-0001_TC-00:00:00_IMG-001.jpg

VHS-0001_TC-00:00:20_IMG-002.jpg

VHS-0001_TC-00:00:40_IMG-003.jpg

VHS-0001_TC-00:01:00_IMG-004.jpg

I am googling now for many hours but i can not found anything to create a filename like this.

Have anyone an idear?

Thx a lot for your help and greating from Vienna

Stefan