LCS is a collection agency, is still in business and cannot "enforce copyright", they can only try to collect from you. If you told them this was a "claim" and not a debt, they are bound by government regulations to cease and desist and send the "claim" back to the client ( photographer, PicRights, etc...), which would explain why you never heard back from LCS.
I would suggest you keep any and all records from 3 yrs ago, in case someone comes out of the woodwork again (picrights), at that time you can politely or not so politely tell them to pound sand, as the 3 yrs SOL has expired...Once the 3 yrs. from the date of their first contact has come and gone, you will be in the clear, unless you continue to use the image, or other unlicensed images..