Lahiri is fairly direct in terms of why Shukumar kept the photo torn out of the magazine.  The narration indicates that "he didn't know why he'd ripped it out."  It was evident that Shukumar was slightly repulsed by the sight of his pregnant wife:

Shoba had been pregnancy at the time, her stomach suddenly immense, to the point where Shumkumar no longer wanted to touch her.  The first time he saw the picture he was lying in bed next to her, watching her as she read.

On one level, Shukumar feels a sensation of sensuality to the woman in the magazine that he no longer feels with his pregnant wife.  Certainly, this becomes one reason why the picture is kept. This  "immense desire he felt for the woman" becomes the base reason he keeps the photo.   On another level, I think that the reason he keeps it is s a reminder of the sensuousness he misses with his wife being pregnant.  It is something that was not purely infidelity, but rather something that he had believed as "the closest he'd come to infidelity."  It is in the idea that there is a sensuous gratification received through the photo that was not being received with Shoba that Shukumar keeps the photo.