The worst image for tourism in Thailand is not from the Africans. The worst image of Thailand is given by all the thais cheating foreigners from anything to a crooked taxi driver on arrival, a dishonest restaurant owner who inflates the bill or gives ridiculously small portions of food or the worst are the building contractors who cheat foreigners during upgrade work in their condos or villas by overcharging, cheating on poor grade materials used, poorly done work or unfinished work.
With of course the usual excuses during the process with likes involving a sudden "break down" of the car (with absurd photos to proove), to the sick family member, or the last minute absence of their employees. Far to many construction service contractors are cheating foreigners openly in places like Hua Hin and this is the worst image that is given to Thailand.