Rob Kardashian -- Please, No Photos! Fled Wedding Over Family Pics

The reason Rob Kardashian ditched his big sister Kim's wedding was simple ... he didn't want to take wedding pics because of his weight gain.

Sources close to the Kardashian clan tell TMZ ... Rob has become so insecure and depressed about his appearance ... he refused to be in the family wedding pics, despite his family's encouragement.

We're told the Kardashians were heartbroken and tried to support Rob, saying he doesn't look bad ... but he still wasn't down to pose