Illusions should be resisted by Magic Resistance. It just doesn't make any sense that you create species (CrIm), but they aren't magical. That's the domain of Momentary Creo Rituals. No, created images should be a magical substance and as such resisted. Likewise species under the effect of Muto.
Now the idea is, supposedly, that you're affecting the process of generating the species by the object. But that's making a major supernatural change to the object, akin to making an animal acquire the mind of a human. Thus, a MuFo(Im) high level effect; perhaps the Im guidelines should be consulted for increases in magnitude and Finesse requirements, but the change isn't in the species, in Imaginem, it is in the Form of the object being manipulated.
So why aren't illusions resisted in the RAW? I suspect it is to prevent the strange circumstance where magi don't notice illusions yet mundanes do. This is a strange artifact, and should be avoided. I suggest, however, that a simpler change be made - that unlike other magical things blocked by the Parma species are still noticeable when they're arrested, they just appear vague, weak, and illusionary. This will allow magi to realize an image is illusory, and avoid falling for illusions if they're protected by MR. A magus wanting to affect another with illusions better penetrate his Parma, just like he would need to with other effects.
This change is far less drastic than the current one, which claims one can use Creo Imaginem to change the way the Animal sheds species - a completely different Form, or even somehow shed natural species out of thin air (without raw vis).