Open your Gallery and the folder you want to delete the pictures from. Now long press on one and it will be selected. If you have multiple pictures you want to delete after selecting the first one just tap the others and they will be selected. Now tap the Trash Can top right and confirm delete.
Yea long pressing the photos you want to delete then pressing the delete button is the easiest and fastest. Another way is to go to the picture you want to delete, press menu button and choose delete.