which is to protect the copyright-holder from unauthorised use of his/her/their property. You're not supposed to remove the watermark, so don't try, especially using the method recommended by woodchip - a five-year-old would be able to detect what you had done.
A final word perhaps- Watermarks are there for copyright protection primarily, as stated but another use of water marks is by recovery/copy shareware for the non purchased version. Essentially of course to get the fee.
Now wobblymike was not specific about the source except to say 'inherited' so assuming they are 'legit' images and all he wants to do is clean 'em up. Then cloning in an image edit package is the only way - not an enviable tast - but do-able