It is desirable to have the resolution of the 2 pictures the same value, although that is not always possible.
- Open the source photo file, make the selection of the object with one of the selection tools, e.g. lasso, selection brush). You should see "marching ants" around the selection.
- Go to Edit>copy to put it on the clipboard.
- Open the destination photo, and go to Edit>paste. The object will be on its own layer.
- Use the move tool to position the object and to resize with the corner handles of the bounding box.
At this stage, I like to put a blank layer between the background layer and the layer with the object, and fill the blank layer with as contrasting color. This allows you to tidy up the edges of the object with the eraser tool, for example. With the marching ants around the object still in place (indicating that the selection is active), go to Select>refine edge, and in the dialog you wil see sliders to feather, smooth, contract/expand.
- Delete the layer created in step #5
TIP: If you need to make the selection active, press CTRL+left click the layer thumbnail of this layer.
The first time I tried this I was practicing and both your method and jon99's worked. Then when I was ready to do the actual intricate selection, I notice that all the layers I had onscreen were affected by the mask selection of the Selection tool. I chose mask becase the selection was large and it was easier for me to invert it.How Is that normal. And some of the objects I had pasted in separate layers came out on the background. One reason for this, I think, is that I didn't notice that I had activated (because it was in blue) one of the different layers I had created. How did I mess this up? How can I not activate a single layer I've created when doing this copy and paste? Do I create a duplicate layer? Do I need to flatten the image first?. How can I start over and not have a single image from a separate layer appear when the first time I tried this the whole, correct selection appeared?
I was typing and did not see 99jon's post. Up to step #5 the methods are identical. Step #5 allows you to refine the selection as indicated so that the transposition does not look fake - adjust edges, feather, etc.
I have difficulty understanding your work-flow. What are you inverting? Why are you using the mask option in the selection tool? When you copy/paste a selection, does it not show up on a separate layer? On the move tool's option bar, top left, uncheck the box to Auto select layer. That may help.
If step #5 is an impediment to your progress, don't utilize it.
Here is a print screen to show a typical layer structure in this type of exercise:
I have the background layer, a cork bulletin board
Layer 1 is a selection of the boats from the picture in the project bin, selection made with lasso tool
Layer 2 is a selection of the tree from the picture in the project bin, made with the Selection brush, mode:selection
Note the checkerboard pattern on layers 1&2, denoting transparency.
I selected the pictures with these colors for visual contrast.
Also note the halo around the tree, as I did not apply step #5 to refine the edge.