Sorry to dominate this thread but I have been doing computer programming for the web for the last 10 years so it's what I do.
SEO and website design....
If it was me, I would not worry about it if it is only a few random referrers. However it could be indicative that someone is using your images and bandwidth and if it grows bigger you may want to move some of your images and other resources on the domain around.
Other than that, I don't think it's a problem...
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