Symphony has some really great image manipulation functions. It's possible to direct a url to a modification of an image, but without actually modifying the image and saving a separate file on the server. It uses this to handle the default images that you can upload as part of a section.

I've got a lot of photography, and I want to display it on the site here, but I don't want to force people to download a 2mb file just to see my pictures. That part is really easy, just change the image url in the Entries component to {$root}/image/740/0/1/fff/{$path}, for an image 740px wide.

I also wanted to use the Lightbox Javascript plugin, so I downloaded that and installed the css, images, and js file. Symphony's CSS interfered with the CSS for the lightbox, so you have to edit the default styles.css and add #package in front of every img ruleset. #package is the default Symphony container div, so you it's just a matter of upping the specificity a little.

Here's an example of this in action...

EDIT: Since writing, lightbox has been replaced on this site.