The iPhone and iPad are great devices for taking and sharing photos, and the iPad's big screen is especially good for slideshows when friends and family are round. (See also: How to put photos on an iPhone or iPad, for more ways to do this.)
It's possible to upload photos from your computer to your iPhone or iPad (or iPod touch, for that matter). The easiest way to do this is to use iCloud.
If you're not already using iCloud to automatically copy your photos from your iPhone or iPad to your computer, you're missing a trick.
iCloud is the easiest way to transfer photos to and from your iPhone or iPad as it uses Wi-Fi: there's no need to connect a cable or sync your device.
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. Using iCloud and Photo Stream is also the easiest way to transfer photos taken on your iPhone to your iPad, and vice versa.
iCloud can also be used to transfer pictures from your computer to your iPhone or iPad. 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.
How to upload photos to your iPhone from your computer via iCloud
(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.
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 do that, either use iTunes and sync them, or install an app which allows you to add files via File Sharing in iTunes.