This block takes the functionality of concrete5's built in image block and expands on it to allow a caption to be added as well an optional lightbox effect. It's great for sidebars and times where you want an image or two to enlarge when clicked, but when a gallery is overkill.
All lightboxed images on a page all end up as part of the lightbox slideshow, meaning you can click forward and back to cycle through all lightboxed images on a page.
The lightbox effect is provided by the ColorBox jQuery lightbox plugin, a very lightweight and well tested lightbox plugin.
There is a choice of 5 different lightbox themes (the ones included with the ColorBox plugin) and these can be easily customised. The theme setting is global, meaning that it only needs to be changed on one block to update the theme selection across your site.
Finally, the image sizes default for new blocks to the previously entered values - this saves time and makes it easier to add blocks uniformly across your site.