After switching my blog to WordPress I needed a simple solution which allowed me to flexibly post images in different sizes without having to hassle with different filenames, paths etc.
The plugin is actually just a port of a plugin/snippet I wrote for MODx a while ago (see here). It scans your content’s source code for <img> tags with width and height attributes and passes them to phpThumb, a great PHP image editing library, which handles the resizing and editing of the image. Additionally, by passing parameters to phpThumb using a query string, it’s possible to edit the image in several ways (take a look at phpThumb’s demos).
The simplest way to get resized images is to specify width/height attibutes in your HTML. Autothumb will use the given values and remove the attributes from your page’s output. When you take a look at the URLs of the following example images, you will see that the <img>-tag’s src-attribute points to a file called image.php in the plugin’s directory. This file takes the image source and several other parameters and passes them to phpThumb, which will generate and output the final image (of course images will be cached).
To prevent evil people playing with the query string and letting your server generate tons of different images, the plugin generates a hash which is a combination of the query string and a secret password you set in phpThumb’s config file. So every call with different parameters (even the same image with different sizes) has another unique hash and it’s not possible to generate new images without knowing the secret password.