Justin Bieber -- Selena Gomez Still On The Brain ... And The Screensaver!

Justin Bieber is not over Selena Gomez ... in fact, she's still very much in the palm of his hand.

We started looking closely at pics of Justin ... and we came across this one (left) of Justin leaving a museum in Amsterdam, and his sorta ex-GF was along for the ride ... as the screensaver on his iPhone.

Justin's background image of choice is clearly a pic of Selena. The sultry pic is from a March 4 photo shoot, TMZ has learned.

Only question is ... did she send it to him, or was he keeping tabs on her?

As TMZ reported yesterday, she flew into Oslo, Norway, to hook up with her on-again, off-again BF.

Now it's clear ... their continuing obsession is DEFINITELY mutual.