Apply transform is necessary Click with your mouse on the background and drag it towards the pasted image. This will blend the background into the image or do vice-versa. To select a part, let s say the sky from a landscape picture, click on the Magic Wand on the toolbox. The magic wand chooses a similar colour. Select Grow or Similar chooses a similar colour so it expands the region of the colour.
Select inverse selects the opposite of what is highlighted.
This is very useful if you have a plain background with an image. Select with the magic wand the background and then go Inverse so that the image is completely selected. This will now select that colour for use.