I’m so torn on this, I didn’t vote on the poll. I can’t choose!
On one hand, I do enjoy “official” artwork, as canon can be a good basis for comparison. Although, sometimes (especially for best selling novels) the authors get very little input into what art goes into the published final product, so you might not even be getting the author’s exact vision.
I also adore fan art because I love that people enjoy a piece so much that they want to express their own mental images creatively! It is very also cool to see how many different people can interpret the exact same characters so uniquely. At the same time, if their version conflicts with what I saw in my own head, it is much easier to dismiss it as their own headcanon without it impacting mine, which is harder to do with official artwork (at least for me, I know that sounds weird).
Along those same lines, sometimes I actually dislike when illustrations are included with a story at all, especially if I have a very solid, vivid picture in my head and for whatever reason the style within the illustrations conflicts with that, it can be distracting. Same with books that have been made into movies…it makes me sad when I start to imagine actors or set pieces when I got back and reread the stories and my own original thoughts become less distinct. Sigh.
TL;DR: My own imagination FTW?