Short answer: Yes, they can.
Copyright holders in the United States have not been required to publish their works with a copyright notice since around 1989. So, unless your infringement started before then, the lack of copyright notice is not a defense to a claim of copyright infringement.
Courts now assume that the average person knows that if they did not create the image, it must belong to someone else and be properly licensed.
The lack of the notice may slightly bolster a claim of unintentional or innocent infringement, but claims reduce liability, they do not eliminate it.
It appears you have a serious problem. You should consider consulting an attorney.