I recognize the artwork, it’s from a well known open source art pack that is used in many game engines examples, and has been redistributed in various formats (if I remember correctly, the source images are in vector format).
The problem with sprites sheets and atlases is that there are many new standards for packaging sprite resources in optimized ways, and some solutions are engine-specific.
So far, the best tool I’ve come across for packing and unpacking sprite sheets/atlases from/to the various formats supported by the different game engines is TexturePacker, a commercial tool by CodeAndWeb:
TexturePacker can also manage color depths conversions, and you can create your custom filters to handle unsupported formats. Being a dedicate tool, it focus on a single task and does it well. It’s very useful for game developers targeting multiple platforms (e.g. Desktop and Mobile) for it can reduce sizes and adapt tilemaps and sprites sheet to different engines (two ways) from a single set of source images.
@a_quack, if you work a lot with third party image assets you might consider getting TexturePacker to handle batch conversions — but the example sheet you provided wasn’t intended for Pixel-Art in the first place, so you’re going to be facing lossy image rsults after conversion.
Usually dedicated Pixel Art game engines don’t use/need texture packers, but if you’re working on modern 2D game engine (e.g. Unity in 2D graphics mode) then you’ll probably have to abide to some packaging standards.
Of course, sprites sheets and atlases packaging could also be done directly in PMNG, but a dedicated plugin would have be to created for each format (since packaging is not an ordinary operation, it might not be worth bloating PM by including it natively). It would be interesting to create a page dedicated to the sprites/tilemaps packaging formats that PM users actually use in their workflow, and gather more info about their specs, which game engines support them, etc. This could be the starting point for considering creating dedicated plugins — it wouldn’t make sense to create a plugin for each format unless we know that someone is actually using them, especially since many of these formats could belong to engines that don’t support Pixel Art well.