When you go out in the sun, you can get sunburnt. Pale skin is more likely to burn than dark skin. Your skin tries to protect itself from the sun by producing melanin, which makes your skin darker. We call this a suntan. The people in the picture have darker skin where the sun has been shining on their skin, and paler skin where it was protected by their shoes.
Sis forSuperimpose
Superimpose means to place one image over another so that both may be seen, often by altering the size or transparency of one of the images.
Surgery is a means of making people better by cutting into them and fixing what is wrong with them inside. Someone who performs surgery is a surgeon.