Complex sentence: Often mounted on the outer walls of pubs, these eye-catching pieces were not printed posters or standard paintings--they were hand-made by teams of skilled commercial artists, many of whom were employed by the breweries for their entire working lives.
Split into simpler sentences: Often mounted on the outer walls of pubs, these eye-catching pieces were not printed posters or standard paintings. They were elaborate craft products created by teams of skilled commercial artists, many of whom were employed by the breweries for their entire working lives.