I don't know why this isn't in the documentation, but someone else answered this here on stackoverflow.
So, you can use this either with a file path string or a stream/buffer, like so:
For a file:
gm('/path/to/animated.gif')
.selectFrame(0)
.write('/path/to/firstframe.png', function(err){
if (err) print(' :( ');
})
For a stream/buffer:
gm(streamOrBuffer)
.selectFrame(0)
.write('/path/to/firstframe.png', function(err){
if (err) print(' :( ');
})
In the documentation they say you need to change the path string to be something like '/path/to/animated.gif[0]'
to be more specific with the frame you want to select, but I tested that code without specifying the frame, and it worked fine (using gm@1.21.1 and ImageMagick 6.7.7-10 2014-03-06). Also, what they had in the documentation (without using selectFrame) didn't work. It ended up crashing and creating several png frames of the whole gif in the process.
Why they haven't documented selectFrame yet is beyond me. As they pointed out in the cited link, there's an open issue wondering precisely that.