Open two images in Photoshop?

I took a portrait on white backdrop paper. I used the new select and mask tool to remove the background so there is the "transparent" checkerboard behind the subject. OK, great so far. But now I want to either fill that transparent background with a color or a neutral gray, or I want to take the original and overlay it onto a separate grey background that I created as an image. But I can't open both images at once to "drag and drop" or find any other way of doing what should really be a simple thing to do. What am I doing wrong? Can you walk me through it? Thanks.