Geulah

I know this sounds REALLY stupid, but I can't figure out which command to use to merge 2 layers. Here's what happens. I open 2 images -- 2 very different photographs, because I'm just practicing. Then -- I want to merge them. But the command "merge layer down" is grayed, and inactive. Likewise the command "flatten". That's the end. I'm stuck and can't go forward.