4:20 PM PT -- It ain't just for looks ... Tim Lincecum is gearin' up for an MLB comeback -- so says the president of Driveline Baseball in Kent, Washington (where the photo was taken).

"Tim will throw for teams at a showcase in the near future," he tweeted.

Wanna know where Tim Lincecum's been for the past year??

Hitting the gym, bro ... and getting SHREDDED.

The former SF Giants superstar resurfaced in a photo from Rockies pitcher Adam Ottavino Monday night ... and it's clear Lincecum hasn't just been chillin' since getting dropped by the Angels last summer.

The 33-year-old's putting his new physique to use too ... windin' up the heater in what appears to be a baseball training facility.

No word on an official comeback -- but either way, lookin' good, Timmy! (We do miss the hair, though.)