You could always do what SI does with football manager mate. Leave the graphics as packs and leave them to the users to create. They can upload and then the people can just install if they want. I mean look at the ribbon app, the new graphics, I also remember the nice planet pack of VB6 for instance. You can leave the blank icons (like they do with the players) so we know what to put where and it's done: No more BS. I understand copyright but it just doesn't make sense for what you do. I mean the poor fella is here harassing for a picture when with all due the respect I bet he just cannot figure out why he cannot produce a decent game like yours with 10 times the people and resources.
I checked and that is exactly what SI does. What is the benefit of having separate packs? If I simply take the images files out of the game download and make them a separate available download, does that remove any issue with copyright because they are not directly part of the game?
No, because the license that the images files use lets you do whatever you want with them and is explicit about what you have to do to be compliant with regards to copyright. Your disney et al images are. . . who knows. Probably not legal to use but it depends on who the original author is. Putting art in a game is not, in fact, fair use. Putting them in a separate pack doesn't actually change anything, because you'd still be breaching 'normal' copyright.
However, and this is important, the prior examples given ARE NOT RESTRICTED IN THE SAME WAY. You can do whatever you want with those ES files because they EXPLICITLY give anyone permission to use them, forever. That license can not be changed retroactively. There are only two conditions that MIGHT apply to you; attribution and making available your changes.
The first is easy, stick a readme in the download, list the files and attribute the author or collective group that produced it. That's it.
The second can seem complicated, but it is not. There's just a serious lack of literacy going around. You need to make available, by whatever means are convenient, the files that you produce FROM the original image file, except for changes to format or to comply with other copyright requirements - like if you had to encrypt all of your images as a requirement to use one of the other images.
What this means is that if you modify the image itself, like changing colors, adding or modifying the image beyond resizing it, you need to have some way for anyone to request that NEW file. That can be my email, download on your site, including it in the download, or even just mailing them a CD.
If you used the image, and created a wholly new image - like a button, it needs to be shareable. If your engine takes the image and just loads it on top of an in-game button? You haven't changed or made a derivative work with the image so you don't have to share anything. Just give credit.
The specific license also means that if you DID make a derivative of the image, it too is licensed under the same form of copyright. Making available would be your only requirement.
tldr; Aurora is NOT a derivative work just because it includes or even modifies the files Steve used. Stop spreading FUD and read the licensing, free and open software already has enough problems without everyone being afraid of working with permissive licensing.