First, enable these backups, which you do by turning on the “iCloud Backup” option in Settings > iCloud > Storage & Backup. Your device will then dutifully perform one daily, but only if it’s plugged in to power and connected to a Wi-Fi network. To force it to do one on your timeline, click the button labeled “Back Up Now” at the bottom of that same window (shown in-progress below).
Choosing that will perform an immediate backup of your camera roll (along with your iCloud documents and settings, text messages, ringtones, and so on), and you’ll even get a handy progress bar with the estimated time remaining. The only caveat is that just like the regular iCloud backups, these immediate ones require you to be on a Wi-Fi network (but you don’t have to have your device plugged in).
Only got 3G in your cabana? No dice. Head out to that local coffee shop and use their network to make sure those wave-frolickin’ pics get saved.