Just don't watch the movie too many times. Eventually you'll forget, and you can go back to imagining whatever you want. This infection of someone else's image isn't a new problem. People have been making realistic paintings and drawings for centuries.
And instead of just bemoaning the lack of imagination, we should think about what is missing when the consumer doesn't supply something himself. I think that it's neat that the technology is advanced enough that Mr. Jackson can show me what he was imagining. But there are things that are difficult to show, and I think that Shelob is a better example of that. After they did a pretty good job with the Nazgûl and the Balrog, I was hoping for more with Shelob. I was disappointed that they just seemed to present her as a big spider. I always wondered how they'd try to convey a sense that she was the "last child of Ungoliant to trouble the unhappy world....who was there before Sauron, and before the first stone of Barad-dûr; and she served none but herself, drinking the blood of Elves and Men, bloated and grown fat with endless brooding on her feasts, weaving webs of shadow; for all living things were her food, and her vomit darkness."
I think that's really hard to convey in a movie, but it didn't seem like they even tried. It's hard to show someone what a complete, oppressive, and malevolent darkness is like. Something that makes your mind forget what light is. Shelob isn't a big spider. She's more like a demon in current terminology: the embodiment of an ancient evil. With good writing and a good imagination in the reader, Shelob becomes much more terrifying than a big spider.