BLENDING TWO IMAGES 

By using the Brush and Eye Drop tool you can sample colours and draw layers to create a rough gradient. Then you can apply a Filter -> Blur -> Gaussian blur, which blends your rough layers and makes a continues gradient. 

These colours will now be visible through the entire image and will indirectly blend two images together.

By using the option shown on the right you can change the saturation, intensity and hue of the previously painted colours and can help change the mood and  warmth of the image to suits your needs.