Alex Rabe's NextGen Gallery is hands down the most popular plugin(by means of total downloads) available for WordPress. NextGen Gallery supports multiple photo galleries, multiple ways to display your photos, and other developers are even creating sub plugins for it! Once installed, you will be able to include your photo albums on both Pages and Posts, as a slideshow (see the following note), image list, or image gallery.
Note
Note: To take advantage of the Flash Slideshow display, you will need to download JW Image Rotator from http://www.longtailvideo.com/players/jw-image-rotator/. Once downloaded, you will need to put the SWF file in your wp-content/uploads directory. JW Image Rotator is released under the Creative Commons license.
Setting up your first gallery
Start off by clicking Add Gallery | Image in the Gallery navigation. The screen will display a tabbed interface, allowing you to add a new gallery, upload a ZIP file of multiple images, import an image folder (located on your server), or manually upload an image one at a time.
Give a name to your gallery. For this demonstration, I named mine Your-Gallery-Name. Then click Add Gallery
Click the Upload Images tab
Click browse and select your first photo
Select which Gallery you would like to put this photo in
NOTE: you will always need to select this for each photo you upload
Click Upload Image
Rinse and repeat as needed
Congratulations, you just created your first photo gallery. Now that we have a gallery with photos, let's insert it into a post. The process for adding a gallery to a post is exactly the same as adding it to a page.
Adding your gallery to a post or a page
Once you visit your Add Post or Add Page area of WordPress, you should notice a new icon available—this is your NextGen Gallery button.
On clicking the newly created button, a new pop-up window will give you the option to select which gallery to include and which method the gallery should be displayed in. Also available from this pop-up window is the option to include an individual photo or an album that contains multiple photo galleries.