There are differant ways, probably the easiest is to:
Take the pic you want to fade and let it overlap the other pic. (if it is white behind it, it will blend threw to white, not into the other picture).
It might be easier to cut out the section of the fading pic that overlaps the other pic where say the truck is to make sure you get all of that cleared out.
Take your eraser and set it to Airbrush with a "soft" brush and lower the flow to about 10% (little more/less.. depends).
Start from where you want it faded the most and work your way out. Let off the mouse each pass of the brush to take another layer (%) off.
A little practice and it will be a nice smooth transition.
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