There are four ways to delete a gallery:
Open the corresponding gallery and click on the three dots in the upper left next to the gallery name. There click on "delete".
In the gallery overview (listing A-Z), move your mouse over the name of the gallery and click on the three dots in the upper left corner next to the gallery name. Then select "delete".
On the right side of your gallery overview, you will find the same three dots at each gallery, if you move the mouse over the gallery. There, too, simply select "delete".
Finally, you can, as always, login into your picdrop account with your FTP program and delete the gallery there.