The typical way to use iCloud is to let it automatically sync photos from your iPhone or iPad to your computer, using what Apple calls Photo Stream. Photo Stream holds the last 1,000 photos, but once photos are in the Photo Stream folder within My Pictures on your PC or laptop they aren't deleted, so it's a great way to automatically back up photos from your iOS device without connecting it via a cable to your computer.
However, you can also use the Photo Stream folder along with iCloud Control Panel to send photos from your PC to your iOS device.
In order to use iCloud on Windows, you'll need a piece of software from Apple. Here's how to install and configure iCloud Control Panel for Windows
Confusingly, there are two different versions of iCloud Control Panel. If you're running Windows Vista with Service Pack 2, download this version of iCloud Control Panel. If you have Windows 7 or 8, you need the latest version of iCloud Control Panel.
When set up, every time you take a photo on your iPhone or iPad, it will be saved to your Photo Stream and uploaded to iCloud, then downloaded to your computer the next time you turn it on.
How to upload photos from your PC to your iPhone
There are various ways to put photos on an iPhone or iPad but here we're going to do it using iCloud Control Panel and the Uploads folder within the Photo Stream folder.
Note that if you merely want to import photos from your digital camera without using a computer, you can buy Apple's Camera Connection Kit if your device has the older 30-pin connector, or the Lightning to SD Camera Reader if it has the new dock connector.
(This tutorial assumes you've already installed and configured iCloud Control Panel for Windows as mentioned above)
Step 1. On your computer, open an Explorer window and navigate to the folder in which the pictures you want are stored. Select those you want, right-click on them and choose Copy.
Step 2. Navigate to the Pictures folder (My Pictures in Windows Vista). Find the Photo Stream folder which is created when you install the iCloud Control Panel, and double-click to view its contents. (Don't click on the Photo Stream shortcut in the Windows Explorer Favourites list as you can't get to the Uploads folder this way.)
Step 3. Open the Uploads folder within Photo Stream, right-click and Paste your copied photos into this folder.
Step 4. Your photos will appear in your Photo Stream (within the Photos app) on your iPhone shortly. The Photo Stream is ordered by the date on which the photos were taken, so they may not be easy to find if you already have the maximum of 1,000 pictures in your stream. However, you may find that Windows gives the pasted files today's date, in which case the photos will be at the end of your Photo Stream.
The images below show iOS 6 on the left and iOS 7 on the right. In iOS 7, Photo Stream is called My Photo Stream and is within the Albums section at the bottom.
Photos you upload this way can be JPEG, TIFF, PNG and most RAW camera formats.
A common source of confusion is that Photo Stream does not support videos, so you can't upload videos to your iPhone or iPad this way. To add videos to your Camera Roll, use iTunes and sync them using the Photos tab (be sure to tick the Include videos box), or install an app which allows you to add files via File Sharing in iTunes.