Images can be a collection (select multiple files in Finder, then click-drag while opening up Dashboard, and releasing within the drop area), or a single image from a numerical series (Sheets will automatically gather the rest of the images for the sequence). While groups of files can contain any number of files and various file names, images are sorted lexicographically - full serial numbers are required for best results (0000 through 0100 will sort correctly, while 0 through 100 will not).
When scaling images to fit within specific dimensions, the scaling algorithm can be selected in the preferences side of the widget. Also new are the alpha output options - transparent (premultiplied), flattened (removed), extracted (alpha only), and multiple files, which produces transparent, RGB, and A files for use in other tools that require straight RGB input (for example, ImageMagick doesn't support straight RGB in the TIF format, so multiple files are the only way to retain all RGB data).
The output filename is taken from the first image in the series, replacing the frame number with a title set in the preferences. For example, a group of images starting with animation_ver1_0000.png by default will result in animation_ver1_spriteSheet.png.
This widget requires ImageMagick to run, please see imagemagick.org for details. In my experience installing via MacPort (macports.org) can take a lot of processing time, but it requires far less user input - just set it up, and let it run for a bit while it installs everything automatically.
Updated on Dec 24 2018