As far as I understand: "kind of". It's very complicated because the license doesn't have any extraneous text.
There's also another complication in that the Artistic License was written for programs, not art, and it doesn't explain what happens when you only use parts of the package. It only gives two scenarios: distribution as a modified "source" and distribution as a modified compiled version. The images are neither a source nor a modified compiled version.
If we assume that images are "source", the license states that I must do at least one of three things. One of these three things is "make the Modified Version available to the Copyright Holder of the Standard Version, under the Original License, so that the Copyright Holder may include your modifications in the Standard Version." -- or, allow the simutrans team to use my images. I would gladly do so, even though my modified images wouldn't really work in Simutrans.
That, however, still doesn't solve the problem of "partial copyright".
PS: Attached is how I am going to modify the images. I've done the steel mill so far. It's refactored to 256x256 + Photoshop blending effects.