Donald Trump Compares Young Photo of Himself to Teenage Barack Obama

Donald Trump's comparing throwback pics against Barack Obama's ... flaunting his put-together military style -- while seemingly roasting his predecessor for lighting up.

POTUS posted a split-photo comparing the two in their younger days ... capturing him in his uniform from the New York Military Academy, which he attended in the early 1960s, and the former prez from early on at his time at Occidental College.

Obama wears a wide-brimmed hat, a collared shirt, and puffs on a cigarette in the pic ... a habit the former president has admitted he's struggled with since his teen years.

While President Trump didn't add a caption to the pic, the message seems pretty clear ... he was strait-laced in his young years -- while Obama was screwing around.

It's just another example of Trump's distaste for Obama ... who said earlier this week he thinks he's basically living rent-free in 45/47's head.

He told Matt Barnes on the "All the Smoke" podcast that Trump doesn't talk trash to his face either ... saying the dude knows better.