I have no doubt that Graal has been subject to copyright lawsuits before, even though I haven't seen the direct evidence. In this instance you're creating a derivative work of what's available at the Kawaii Sister's website. The right to distribute a derivative work based on an original work is under the precipice of copyright law. You would need to provide explicit evidence of permission to create and distribute a derivative work in order for it to be cleared.

If you can make the argument that rejections of custom graphics are decreasing Graal's potential revenue, then you have a completely valid argument to suggest that Stephane subscribe to a copyright clearing house if he hasn't done so already.