itch.io

I totally get where you're coming from. Most source code licenses come with the restriction that copyright notices and they themselves need to be replicated in order for them to apply, I assume for exactly that reason. I was asking, because your new Rules.txt (as of Natsumi) carry the line "If re-uploaded to another site for download, do not remove this text file or any part of it." In my rewritten rules, this is formulated as 

The above copyright notice, as well as this permission notice shall be included in all works, that redistribute this artwork partial or in total, with or without modification.

which would force games, that use your assets to carry that notice. (Now that I think of it, the phrase "work, that redistributes this artwork" is perhaps a bit unclear. Does it apply to all redistributions? Does it only apply to tarballs/zips?) The reason for that being, that if someone decided to fork an open source project on itch.io, that included your artwork, they wouldn't necessarily know that your artwork is being reused and which terms apply to it otherwise.

For what it's worth, you could also license your artwork as a whole under CC BY-NC version 3.0 with an exception notice like the following:

As an exception to the conditions laid out by this Work's license, You may commercially Reproduce 
or Adapt the Licensed Material as part of a game, a visual novel or any other comparable interactive
medium (henceforth referred to as "Game") subject to the following conditions:
a. You must retain a copy of this exception notice in addition to the material covered by 
   section 4(a) of this work's license.
b. Your Game must list the Original Author by their name among any credits that your Game displays
   during execution.  If your Game would normally not display such credits, you must provide means
   to access them from within your Game.
c. To the extent that it can be reasonably expected of You, You must inform the Licensor about Your
   usage of their work for commercial purposes prior to or at the time of the publication of your Game.
   If the Licensor deems, that Your usage does not qualify for the terms of this exception, you must
   at once cease all commercial use of the Work in your Game.
d. You must otherwise follow the conditions laid out by this Work's license in the manner that they
   would apply to Reproduction and Sharing for NonCommercial purposes.

The CC BY already covers many conditions of your rules, those being:

1. Requiring attribution
2. Not allowing illegal themes or themes that would hurt your reputation [section 4(c), which IIUC is not present in version 4.0]

CC BY-NC additionally adds, that you cannot reuse the work commercially (without dual licensing). With the exception notice above, commercial use in games is allowed as long as credits are made, the exception notice is kept and an attempt is made to inform you, e.g. through itch or deviantArt.

Not covered by this would be non-commercial redistributions of your assets, which remove the exception notice. I.e. it would legally be possible to redistribute the work under CC BY-NC without adding the exception notice. I don't think that's too big of an issue, however. Also not covered is the kind request "Not required, but I would love for you to link me to your project!" as it is not legally binding.

For this license package to apply, you would have to add three files to your packages:

1. The CC BY-NC license text. (Names for that should be COPYING or LICENSE)
2. The exception notice (similarly named COPYING.exception or LICENSE.exception)
3. A file with crediting information of the covered work (a suitable name would for instance be CREDITS).

The CREDITS file could for instance look like this.

Natsumi Character Sprite <https://noranekogames.itch.io/natsumi> by NoranekoGames (@NoranekoGames, noranekogames.itch.io)

You can also add comments like "I would love for you to link me to your project!" into it, if you want to, but I'd recommend keeping it clean and simple.