You just ran into the problem most new users have with Android as a mobile operating system.
its not intuitive.
no easy way to tell how to do basic tasks unless youve already done them before.
basically when your in the pile view (where pictures are grouped into "piles") tap a stack of pics to expand it into the view where you can see them all.
then LONG press any picture and youll see all the pictures have a small checkmark over them. after the one you long pressed is highlighted, you can tap any others and highlight them as well.
once you selected the ones you want, there is a delete button at the bottom of the screen. this delete button will be here since you started the long press, so youll have noticed it by now.
hope this helps.
also, im pretty sure the LG instruction manual that comes with the phone mentions this, as long as your using the stock ROM. i remember skimming through it and it seemed pretty thorough on basic functions like this