Simply Flag the photo(s) you want to move. You can do this by clicking the little Flag icon when hovering over the photo, selecting the photo you want and using the Photos > Flag Photo menu command, or using Command + . (period) shortcut.

Once you have all the images you want to move Flagged, just click on the Event you want to move them into and go back up to the menubar and choose Events > Add Flagged Photos To Selected Event. Or if you want to create a new Event to place your Flagged photos in, choose Events > Create Event From Flagged Photos.

Once you’re all finished moving your photos, you can unflag them by selecting them and hitting Command + . again.

I’ve always just used Command + X and Command + V to cut & paste photos from one Event to another, but this method seems a bit safer if iPhoto crashes before you paste.

Thanks to MacObserver for this tip.