okay... so I finally got a render last time of a pikachu by saving it and then opening as new layer and it was transparent

Now when I am trying to do it with other pictures, they aren't transparent.

How do I do it again?

it must originally have had a transparent background for it to open that way
if they aren't, you need to manually remove the background
was your pikachu one already transparent when you saved it?

Make sure you're saving as .png