NBA's John Salley On Usher Botched Penis Pic ... Take My Advice ...

Heads up ... there's a lot of genital talk in this video ...

John Salley has some "pointers" for Usher about how to take a world class ... Internet-breaking ... truly inspiring and memorable ... picture of your penis.

Usher "inadvertently" showed his little guy on Snapchat Thursday ... posting a nude snap with a smiley face covering his happy place ... but leaving a little bit left over to say hi.

We asked John about this man-sized malfunction when we got him out at LAX -- 'cause he's sorta like an expert -- and he gives us the lowdown on proper penis pic etiquette.

Take notes, fellas ...