The best way would be to download the sprite sheets (from opengameart.org) and then write your own sprite sheet generator. The online one has issues especially with oversize weapons and some of the sprite sheets are incorrect or missing and not in the generator. A generator like that is not hard to make .All the generator does is layer the images on top of each other, and it supposedly changes their color. But, I could not get the color changing to work. If you're writing an open source game construction kit you should most likely already have image / sprite layering implemented. So writing the generator should be trivial. I'm writing one for my engine.

If you crack open the source code, you'll understand the issue immediately. All of the data about the different layers/sheets, logic about what should be mutually exclusive, Z-order, etc. is baked into the HTML, or is in code tightly coupled to the HTML---there's no library function you can call that will assemble a list of items into a sheet for you. There are a handful of weird special cases with specific items (certain hats overlap weapons; there is code to handle some of these cases, but not all); again, the code to handle this is tightly coupled to the HTML interface.

Attribution is also a big problem. The current generator contains lots of unattributed images. (Technically, they are probably all attributed in the CREDITS.txt file, but lots of things are renamed, rearranged, or recolored, so it's really hard to find where something came from after constructing it in the generator). Castelonia recently started adding attribution to the HTML file so after generating a character, you can download a list of attributions for the different parts, but this is not complete yet.

IMO the generator could use a clean re-write, ideally into a form where there is a separate Javascript library that can be called to generate sprites from a list of layers, handling all the special cases and so on. I've discussed this briefly with castelonia but don't really have the bandwidth for it now.

I'd be willing to rewrite the generator as I'm kind of doing that already. If I did it though it would be using libgdx / java / springboot but still on the web. It would also have the option to pack of the assets into a libgdx texture atlas, and allow logins where users can save assets / sheets they've created . It would also let users recolor the sprites but, how do I handle re-coloring sprites legally?

What do you mean? What is the legal problem?

When I referred to things being recolored as an attribution problem in my original post, I meant that the recolors sometimes made it confusing to find the original author of an image. I think recolors are fine as long as the original is properly attributed.

My ideal version of the generator would handle attribution like this:

- When selecting layers, you can mouse over/right click/click a button or something to see the author, license, and link to the original submission on OGA or similar
- When you make your sprite, there is a button to "download credits" for the individual sprite, which would have all that information for the layers you actually used
- There would be a master credits/attribution file, ideally CSV/JSON or similar that you could download which contained author, license, and link for every file available in the generator