One of the best moves that the PHP language developers could have made was to bring the GD library code "into the fold" and integrate it with recent PHP installations. This makes it easy to manipulate images and customize things on the fly. Another thing it allows is the generation of thumbnails for your site, either dynamically or in something like a cron job. Want to know how? Well, read up here.
In this article from Icant.co.uk, they step you through the creation of these smaller, lighter versions of your images with lots of code and explainations. Topics covered include:
- The logic of batch processing thumbnails
- The PHP functions to use
- The main thumbnail generation function
They also include instructions on how to check your installation for GD support and how to load it if it's not already there (module support).