A plugin plus a component
Before, ScriptMerge used to be just a plugin, but now it ships with a component which is required for the correct workings of the plugin. The main reason is that ScriptMerge now offers two different Merge Types: A cache-file which is written to the Joomla! cache-folder, but also a dynamic list which gets built every time a page is called. The first could be seen as static - good for performance. But the second is actually good when Joomla! page caching is applied, but still the CSS could dynamically be changed: The WebP trick is one of these dynamic conditions.
ScriptMerge now has WebP support - a format that allows you to replace PNG and JPEG images for specific browsers (Chrome browsers have native WebP support, Firefox has a plugin for it), which will reduce the download-size of images dramatically. When WebP-support is detected, ScriptMerge is able to replace all normal images with WebP images. But because this is changing the CSS dynamically, the static cache can not be used: Instead the component is called upon. Note that you need to have WebP installed on the operating-system level to use this feature - if you're not a sysadmin, forget about it.
Data URI support
The new ScriptMerge also supports data URIs - small images can be included in the CSS itself as data URIs, without the need for the browser to make an extra HTTP-call. The maximum size for images served this way is configured in the plugin.