I'm kind of new when it comes to Photoshop, so this question may sound silly to about 98% of you. ^_^
Anyway. How do I put a 100x100 image (that looks like a gradient/texture/brush/etc) as a new layer on my icon? I have no idea how to do that.

Also, is it possible to flatten only 2 layers when you have more than that? If so, how?

I use Photoshop CS.