Justin Bieber -- Lighting Up ... In Possible Weed Smoking Pics

The photo has been all over social media today ... showing Bieber sitting next to a girl that looks a helluva lot like Ariana Grande -- and he appears to be holding a long joint.

As you may recall, Bieber was photographed similarly back in January ... when he was seen holding what looked like a joint at a Newport Beach party.

Sources close to both singers say the girl in the photo is NOT Ariana Grande -- and the pic was photoshopped to look like Bieber was lighting up.

However our crack art department begs to differ -- saying the light source seems consistent ... and any indicators of a doctored photo are non-existent.

Keep in mind ... our art department also made this.