When iPhoto imports images from your camera or iPhone, it decides how to set up new Events based on what’s configured under iPhoto> Preferences> General> Autosplit into Events.
If you’d like to split your Events up more finely than that setting allows, though, it’s pretty simple. All you have to do first is double-click on the Event you need to edit. Once you’re in, select the first image that you’d like to move to a new Event and choose Events> Split Event.
That’ll place the photo you selected (and everything after it) into a separate, untitled Event.
On the other hand, if you’d rather split off some images in the middle of an Event, you can do that too. An easy way to select the ones you want to move is to click and drag a box around them, because as you drag, every image the box touches will be selected.
When you pick Events> Split Event afterward, you’ll end up with three: the one you started with, an Event containing the images you selected, and an Event with all of the photos after those you selected. Easy!
Note that if you want to move the images you’ve picked to a new Event rather than splitting an existing one up, pick Events> Create Event instead after you do your selecting.
That’ll add an Event for the chosen pictures, but it won’t affect the existing organization otherwise (i.e., you won’t end up with a new Event for the images that appeared after the ones you moved).
When you’re finished getting everything just the way you want it, be sure to click where you see “Untitled Event” to give your new creation a name.
I know that’s not a very creative name, but to be fair, they’re not great photos. Hey, when you take 10,000 pictures of yourself sticking out your tongue, it’s difficult to find a few in a row that are publication-worthy, you know?