If you're using PSE 11, you must go to the editor preferences>general and turn on Allow floating documents in Expert Mode, then to window>float all in windows, if you want to have several image windows open in the workspace at once. Or you can use the layout menu at the bottom of the screen to arrange image tabs, if you prefer them.
I opened a new A4 sheet, then a Sky image, then an image that needed a
replacement Sky. Each opened as a background image and the other 2 Layers
couldn’t be dragged to the Layers Palette from the Photo Bin.
I hope this makes more sense.
Open all the images then select the A4 sheet by double clicking on it in the photo bin or create a new blank file.
Then drag each image separately up into the main window (e.g. drop on top of the A4 sheet)
Aeparate layer should be created automatically for each image.
You can open both images and have them on separate layers, but the top layer is the one visible on screen at full layer opacity. In order to see part of the layer below, you need to "punch a hole."
In this example there is a sky picture for the background, and a picture of cabanas with a drab sky above this.
I used the magic wand tool to select the drab sky, then deleted the drab sky, allowing the "new" sky to to be visible..
You can do this with a mask as well.