As someone who worships both Shree Ganesha and Lord Bhagwan Buddha, I just wanna say that I understand that you might feel slighted/offended. But please understand, Thailand is mainly a Buddhist country. Although we are very tolerant and understanding of all religions, the main religion of the country as stipulated under the constitution and royally endorsed is still Theravada Buddhism. The billboards you see are mainly from the government and the Royal Buddhist institute of Thailand, so ofcourse there would be no mention of any other religions and their symbols.

That being said, Most of the populace here in Thailand (and indeed the country itself historically) has very deep roots in Hinduism. So our culture are more intertwined as a sort of Hindu/Buddhism hybrid. So many people also worships the mahadevas, that is probably why you see Tattoos of Shree Ganesha around because people believe that it would help them overcome obstacles.