Everyone's done this at least once. You take a photo with your new digital camera and then you post it to your web site. You can't wait to see your picture on the site and then it's way to big.
Here's the deal. Pictures have two characteristics you need to consider if you're going to include them on your web site.
1. Physical file size: How big in bytes is your picture. Kilobytes are fine, but Megabytes are not so good. If your picture is larger than 1 megabyte, consider resizing the picture. Otherwise, your website will appear to slow down as your visitors wait for your huge pictures to download.
2. Dimensions: The width and height of your picture in pixels. If you don't know what a pixel is, think of pixels this way. The average website is probably around 960 pixels wide. If you want your picture to take up 1/3 of the page, your picture should be 320 pixels wide (960 / 3 = 320).
This video illustrates how to use Photoshop and Ittrium to resize your photos.
This video shows how to use Ittrium layout features such as the Grid to resize your photos on the fly.