In order to scan, restore, print or otherwise alter or duplicate a photo not taken by me but supplied by a client, I need to know that either:
the client holds the copyright to that photo, or
the client has permission for that use, from the copyright owner, or
the image is public domain or copyright has expired.

This is seldom a problem for informal family snapshots, but may be an issue with formal studio portraits, particularly if taken since the middle of the twentieth century. Check to see if there is a studio logo or copyright notice on the photo & obtain permissions from them if possible.

Clients for such services will be required to sign a "hold harmless" agreement certifying that they have such rights, & indemnifying me from liability in any copyright issues surrounding the photos. (This agreement may cover multiple services and so need only be done once, up front.)