I don't really have an opinion on the licensing either as long as I get credit for my designs and they won't be altered (after the final look has been approved by you guys).
The BSD3 license (see the table at
https://choosealicense.com/appendix/ as well as the license text at
https://choosealicense.com/licenses/bsd-3-clause/) allows modifications (even without mentioning the modifications). It requires giving credit and prohibits showing endorsement by the original author for any derived works. The whole of OpenMPT (with the exception of external libraries that we use) is licensed under this license, and even though application of this license to artwork is somewhat difficult (I Am Not A Lawyer, though), I do not think we would want to license certain parts of OpenMPT not (at least also) under this license.
The CC-BY-4.0 license also allows modification, however it requires the modifications to be clearly mentioned. It also requires giving credit to the original author and also prohibits showing endorsement by the original author for any derived works.
See
https://creativecommons.org/licenses/by/4.0/.
There is also a CC-BY-ND-4.0 license (
https://creativecommons.org/licenses/by-nd/4.0/) which would prohibit all modification. However, given that BSD3 already allows it, I doubt that would be of much practical use.
Also, use on Wikipedia would be impossible (see
https://commons.wikimedia.org/wiki/Commons:Licensing#Acceptable_licenses) under CC-BY-ND-4.0. I am not sure if BSD3 itself is disallowed on Wikimedia Commons. However, an explicitly allowed license would certainly be preferred. An acceptable CC license would make things way easier and clearer (which is precisely why I came up with the suggestion of dual-licensing the artwork in the first place).
CC-BY-SA-4.0 license (
https://creativecommons.org/licenses/by-sa/4.0/) would require any modifications to be released also under the same CC-BY-SA-4.0 license. As far I am concerned, I think, dual-licensing under BSD3 and CC-BY-SA-4.0 could also work for OpenMPT artwork in practice, even though the spirit of CC-BY-SA-4.0 is more like the spirit of GPL-style licenses and less like the spirit of BSD-style licenses which OpenMPT uses. I would still highly prefer not to have this restriction on the secondary/other/dual-license for the artwork, as it makes the 2 licenses rather inconsistent in meaning and spirit.
In any case, consider the following:
If someone (for whatever reason) does some modification to OpenMPT and distributes it, he/she is certainly allowed to do so by the OpenMPT license. Even though we would like the name and branding to be changed in that case in order to avoid confusion, this is not required by the license itself. If the artwork license would prohibit any modification, anyone modifying OpenMPT would either need to stay with the exact same original artwork (and thus would be forced to give somewhat wrong impression) or to use completely new and unrelated artwork all together (which will force anyone who wants that to do more work). (Something similar (though not identical, something trademark related was also ivolved AFAIK, I will not try to go into legal details here, because I Am Not A Lawyer) to that happened for the Mozilla Firefox branding which made it impossible for the Debian Linux distribution to actually name their firefox browser "Firefox" and use the commonly used firefox logo. They had to invent a new name and logo).
I am aware of the fact that designers tend to want to never ever see any modifications to their work by others (I can even understand the reasons why), however that mindset kind of conflicts with the principles of open source software. I do not expect us to randomly change your design just because we feel like it; that would be highly unlikely to happen. However, I do not think we could accept contributions with this restriction written into the license, as that would make modifying and distributing OpenMPT harder. It would even prevent us from changing something as simple as the dimensions of the logo background, even if required for any particular use context. Also note that any specification or limition with respect to who can modify the design would also be unacceptable, because change in maintainership of open source software as well as forks or otherwise derived software could happen any time and are the foundation of how open source works.
If our license requirements conflict with what you are willing to license your work under, we would very sadly have to reject your contribution.
