To use two images already posted by QUIB--
1) open the AR15 pic.
2) make another layer over it, and copy&paste the helicopter pic into the new layer
3) select JUST the helicopter pic/layer and then inverse the image (it should now be a white helicopter on a black background)
4) slide the helicopter pic over a dark part of the rifle image
5) change the helicopter's layer blend mode to "lighten" (the black background should disappear now, leaving only the white parts visible)
6) adjust the helicopter-pic layer's transparency down to 20% or so.
7) flatten the image into one layer.
.....
The rifle pic given is rather a poor example; ideally you want the image to be very-dark colored with a lightening watermark or a very-light image with a darkening watermark. Or with this one, you could add in a darker background as the base layer, and set the rifle layer (#2) as "darken", and adjust the background's layer until it is rather light. And you often have to put a layer behind the foreground object and paint white behind it, to keep it from disappearing into the dark background.
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