Once you upload the picture, the picture record is saved on the Picture table and then the mapping is created with the product and the record is saved on Product_Picture_Mapping. So, you would need to remove records from both the table(Will work if you remove the record only from mapping table too but there will be unused picture records on picture table).
You can use the following SQL command to remove the picture from the database.
Delete from Picture where id in (Select pictureid from Product_Picture_Mapping where ProductId in(1,2,3,4))
This code deletes the picture stored in the database. Make sure you replace the productid(1,2,3,4) with the product id whose picture you want to delete. This automatically deletes the product picture records from product_picture_mapping table too but you can run the below query to verify the record is deleted from the mapping table
Delete from Product_Picture_Mapping where ProductId in (1,2,3,4)
Make sure you backup the database before you execute the query in the production environment.