Hi konstamax,
If you are in the backend go to the gallery where the comment has been made.
find the picture where the comment has been mede you want to delete.
On the right next to the picture below the ‘Activate/Delete’ tic-box,
You should see ‘Comments’ right above above ‘Go Save” click it.
Then you see the comments made..
Behind every comment there is a ‘Delete’ tic-box.. tick it, and click the ‘Delete’ button below..
That deletes the Selected (ticked) comments..
(if you see no ‘Comments’ link, no comments have been given on that picture)
I hope this answers your question.