Vintage comic art, which actually is not Boba Fett, but a Mandalorian. The text on the shirt does say Boba Fett, thou.
From the manufacturer:
You've never seen Boba Fett like this. The Star Wars Boba Fett Graphic T-Shirt features the galaxy's most ruthless bounty hunter in a retro-style illustration that brings in an element of nostalgia. Made from soft 100% cotton for maximum comfort and breathability, this shirt will complete any casual outfit. When the Empire Strikes Back, you'll be ready.