"The artwork is under GPLv3 license." as you write yourself.
But GPLv3 is a copy-left license and can not be used with a MIT license.
You can use also the GPLv3 license for the mod or you could the sentence "The artwork is under GPLv3 license." just delete and hope that nobody notices until you have your own graphics ;-)
In general, using MIT is perfectly fine, but you need to pay attention to what licenses the components you are using have.
With this notice I just wanted to draw your attention to the tiresome topic of correct choose of license.
best regards
Kux

Thanks for pointing this out Kux. As it turns out I had a typo in my acknowledgement. Miniloader (and the graphics) are released under LGPLv3, not GPLv3. Granted "Not a Lawyer" but I believe one of the distinctions between LGPLv3 and GPLv3 are that with LGPL derivative works are not required to be released under the same license. I have corrected the acknowledgement.

LGPLv3 is OK. LGPLv3-licensed components can be used in an MIT project as long as the components remain under LGPLv3.

As it happens, i made the art, not Angel. They're a cut down version of the art i made for Vanilla Loaders, which is MIT.

By which, I'm happy to contribute them to this mod as well under MIT.

Awesome.. I was starting to think that was the case as I started looking at prismatic belts. Thanks Kirazy. I'll make sure I get that acknowledgement updated for the next release as well.

ROFL that so far to ‘hope nobody notices until you have your own graphic ;-)’
Honestly would be the longest.