Photoshoot is a compound word in the making, but it is not yet fully accepted. For as long as the words photo and shoot have been commonly used together—which is only a couple of decades—the two-word form, photo shoot, has always been the more common by a large margin and remains so today. The hyphenated form, photo-shoot, began to catch on fairly early, but now that form is surpassed by photoshoot, which, as of 2013, is fast gaining ground.
In current searches of news publishers with content available online—these tend to be at the forefront of usage trends—photo shoot still appears about three times for every two instances of photoshoot. Photoshoot was almost nonexistent just a decade ago, though, suggesting that English’s compounding impulse is working fast in this case.
Of the main parts of the English-speaking world, the U.K. is most accepting of the one-word form; it now appears about once for every instance of photo shoot. The one-word form is still relatively rare in the U.S., though scattered instances are easily found in edited texts of the last couple of years.
The hyphenated form, meanwhile, is almost nonexistent in 2013.