The New York Times explanation:
In the four-missile version of the image released Wednesday by Sepah News, the media arm of Iran’s Revolutionary Guard, two major sections (encircled in red) appear to closely replicate other sections (encircled in orange).
If you look at the third photo, it appears that one of the missiles failed to launch, and then they tried to cover that up in the doctored photo.
(Illustration by The New York Times; photo via Agence France-Presse)
This final image, part of a framegrab from an Al Alam television broadcast of the launch (apparently taken at approximately the same time from a slightly higher elevation), makes it pretty clear which missile was conjured from borrowed pixels.
So, Agence France Press (AFP) had to retract it, saying:
A handout picture released on the news website and public relations arm of Iran’s Revolutionary Guards … shows an image apparently digitally altered to show four missiles rising into the air instead of three during a test-firing at an undisclosed location in the Iranian desert on July 9, 2008. The 2nd Right missile has apparently been added in digital retouch to cover a grounded missile that may have failed during the test.