The USPS says in their press release that:

The Container Ship stamp is based on an undated photograph of the R.J. Pfeiffer, a modern container ship launched in 1992 and operated by Matson Navigation Company. Container ships, pioneered in the 1950s, are key to today’s global economy, carrying manufactured goods worldwide across the oceans and exemplifying the modern merchant marine.

After a cursory search, it appears that the stamp’s illustration was based on a photo from the International Organization of Masters, Mates & Pilots website, bridgedeck.org.