According to the book Universal Principles of Design people have a tendency to view attractive people as more intelligent, competent, moral, and sociable than unattractive people
This bias appears to be inborn. An evolutionary explanation would say that we have a drive to reproduce because reproduction passes on our genes. Healthier mates increase the likelihood of passing on our genes and attractive people look healthy. These explains why men are naturally attracted to women with symmetrical features, a hip-waist ration of 0.70, and exaggerate features of health and sexuality (by wearing makeup or push-up bras). Women are naturally attracted to men with a hip-waist ratio of 0.90 and who show signs of wealth or power.
Looks are very influential as mentioned in my previous post. A perfect example is the first presidential debate (1960) between Richard Nixon and John F. Kennedy. Nixon was ill and running a fever, he also wore light colors and no make-up. Kennedy wore dark colors and make-up. Those who heard the debate over the radio agreed that Nixon won the debate, but those who watched it on TV agreed that Kennedy won the debate.
The use of photography of people is very a very effective advertising and marketing tool. Use attractive people that look healthy. But, if you use photography that is too sexy, you will fall into the intrusive death cycle of advertising and your ads will be less effective in the long run.