How do I get a picture to show the picture below it? Not like with adjusting to opacity or changing the layer's style to "color" or whatever, but like blend the two images without losing the integrity of either.

Like I saw a tutorial of it once where there was a crumpled piece of white paper and they put a photo of a forest on it to make it look like it was a crumpled picture of a forest. I thought I saved that tutorial but I seem to be unable to find it.