The sentence in the contract can be interpreted a couple of different ways.
- The most obvious interpretation would construe the clause to mean that ownership and copyright of images produced after the rental period by the said equipment would be held by the lending company. This would certainly deter the borrower from continuing to use the equipment after the rental period. Under this interpretation, the rights to images produced during the rental period are not affect.
- I think it would be a flimsy interpretation to read the clause to mean that if the equipment was not returned on time, then all ownership and copyright of images taken during the rental period would revert to the lending company. This would accomplish the deterrence goal, but this meaning isn't clear in the plain reading of the sentence. It would also mean that all borrowers who have ever returned equipment a day late have all signed over their ownership and copyrights of images and it's a backdoor way for the lending company to obtain image rights. Courts aren't likely to accept that outcome.
The lending company is interfering with your right to recover payment for your services if they reached out to the publishing company claiming copyright issues. That would appear to create direct monetary damages and reputational harm to you.
Considering getting legal advice if this carries on. Sorry you have to deal with this issue, too.