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Open your image. First select Image/Mode/RGB and then select the Magic Wand tool, select the white background and press the Delete key on your keyboard.

Drag this image to a document that already contains a background.

Move the copied image around or resize it by selecting Show Bounding Box on your options bar (just beneath the menus)

Drag one of the corners while holding the Shift key to resize your image.

Adjust brightness of the main subject.

Here's a quick result;