Thanks for the suggestion Doitsu.
I do have the python3-pil installed. Maybe the fact that I'm not using the bundled version causes the plugin to not find it?
Edit:
I believe the problem is around my Python installation. I seem to have both 2.7 and 3.4 installed and even though Pillow is installed it would seem it's only available to the 2.7 installation. If I remember correctly it's 2.7 that came along my distribution and I had to install 3.4 manually to get Sigil to compile. Since everything has been working fine I haven't thought about this (don't know Python). Guess I will have to sort this out now.
Thanks again.