For combining images in Photoshop I always use the Merge option.
-First, highlight/Select+Hold Shift all layers you want to combine.
-Then right click or Control Click the selected layers. You will see a long menu open up.
-Scroll all the way down and you will see the Merge Layers option.
This option makes the images selected into one image and you end up having a single layer. I would suggest before doing that to make a copy of each layer you want to combine and put it in a folder layer. If you want to go back and change something, this will be easier for you to get back to those original layers and make your changes. Hope that helps!
