A cutline is simply the text below a photo.
Newspapers, blogs and websites simply need to identify who's in the photo and who took it, which they call a "photo credit."
The easiest mistake is to write a cutline that is far too long.
Unless a photo is going out as a standalone, you don't need to explain too much. They'll have the press release, oped or other material that goes along with the photo. All they need is a little context and to spell the names right of whoever's in the photo.
A cutline is written like a straight news piece, with a sentence or containing the Five W's: who, what, when, where and why.
Cutlines don't need to be artistic or creative.
Most of the time, a newspaper or blog will rewrite the cutline, depending on whether the photo needs to get cropped down to fit their layout or can get stretched out to fill space.
Caution: Cutlines must include a photo credit. Newspapers and blogs are sensitive about not printing photos that don't have a photo credit -- they don't want people sending photos they don't have permission to publish. Make sure you've got the right photo credit, and that you include it in the cutline.