Wait... what? The MIT license applies only to the
illustrations? The MIT license is specifically a software license and doesn't really work for illustrations, except to the extent that they make up the software's associated documentation. Can you elaborate what this means? I would have expected exactly the opposite: that the code is MIT licensed and the illustrations are under your custom license. The reverse is a very bizarre arrangement.
It seems to me that the MIT license isn't really involved at all here. It can't apply to standalone illustrations, because MIT is a software license, and you're not open sourcing the code. What, specifically, is MIT licensed here?
For illustrations you want something like Creative Commons. Those licenses are not software-specific like MIT is.