Age and sun and a little extra weight.
The cheekbones are the same.
The nose also?
Ahmadinejad's nostrils are huge. An especially good example is the picture in the top row, middle. Check out the bottom row, middle, for the size of the nose in general.
I'm not seeing much reason to think it's him.
The fact is, all these guys sort of look alike, and so far in that picture of the hostage being escorted I've seen all three guys on the right identified by somebody or other here as Ahmadinejad. It's not evidence of his presence there so much as it is evidence that Middle Eastern faces have a lot of generic similarity.
He could have been there --- it hasn't been established that he was or wasn't --- but I don't think any photographic proof has yet been produced.